Relating to Homeseer 3 there are two UPB plugins available. (and 3-4 scripts / plugins relating to the use of the Amazon Echo).
Homeseer 3, Elk M1 plugin, UPB and Amazon Echo (and now Google Home) all work fine and easy together.
Why work and do this the long math way? As mentioned above Homeseer 3 Zee is available today for less than $100.
The limitation to Homeseer 3 Lite (Zee2) is 5 plugins and today Homeseer Touch and Amazon Echo integration is included in the core HS3 OS.
The ARM CPU architecture is quite different to a x86 CPU Architecture so any comparison is largely meaningless.
Though as we are playing fantasy would say running Windows XP on the last of the Pentium 4 Socket 478 CPU is a "reasonable" comparison when undertaking Web Browsing and Office Suite tasks in the latest Raspbian Jessie with Pixel.
The Homeseer 3 lite Zee2 program will run on any Linux computer with Mono installed. It is not just an RPi Wheezy or Jessie program. IE: Homeseer Zee2 will only run on Linux / iOS while Homeseer Standard / Pro run on Windows, Linux or iOS.
For granular tinkering with the Zee2 you should be familiar with Wheezy or Jessie and Linux and the Homeseer 3 program. Homeseer 3 runs using Mono for Linux. You do not really need to be familiar with Mono rather just Linux.
For scripting using Homeseer 3 you can utilize just about anything that runs in Linux plus native Homeseer 3 VB scripting. (here also utilize bash, perl and python). Native too are some commands that you can compile from C right on your RPi.
The free one was never really updated much and the paid for UPB plugin which works fine.
If you have an OmniPro 2 you can talk to the UPB via an Omni Plugin command to a unit and or using the two UPB Homeseer 3 plugins.
Adding the HAI Logger to the mix you can also utilize the Samsung Smartthings hub.
IE: here have multiple UPB PIMs configured. (Homeseer 3 (2), remote serial -1 , OmniPro 2 Panel - 1) mostly configured to tinker with and check signal strengths of UPB (no issues today).
Relating to using Homeseer 3 I have configured remote RPi's with a ZWave GPIO, X10 CM11A and UPB serial PIM; all work fine using ser2net or running the plugin remotely...IE: mono homeseerplugin.exe server=ipofhomeseer.
Currently have a POE connected RPi2 in the attic with a ZWave GPIO card which talks to Homeseer. Testing it is also running Domotize and a 1-wire network covering the second floor and attic temperature and humidity sensors. Introducing another POE RPi2 with a Zigbee GPIO card to a bit more stuff.
I have no issues here using cheapo usb to serial devices, Digi Edgeports or a couple of EU USB to 4 serial port devices. Doing this today for my 1-wire hub. I see some stuff above using direct pins for a TTL connection to the RPi. You can purchase a USB to serial cable or connector today for less than $10. For the Homeseer stuff been mostly using Digi Edgeports, Digi USB Anywhere devices, Digi USB powered hubs...now for over 10 years with absolutely no issues. IE: the big Homeseer 3 Pro mothership running headless is using one USB cable to one 7 port Digi powered hub to 5 USB devices and 2 Digi edgeports and works great. Audio (multiple SAPI voices - VBs) is one wire to the the Russound system.
Here have updated the RPi 2 Zee2 computer to running on a Pine64 2Gb machine with Ubuntu 16.04 64bit versus running Wheezy / Jessie or Stretch on the Zee2. (under $50). It has same pinouts as an RPi2 and includes an RTC. Next update is to the Rock 64 4Gb machine running OS on an eMMC versus a microSD. Also tested and working well was the Xi5a cube computer running with a dual AMD CPU, 2Gb of RAM and a 32Gb SSD drive. It is larger than an RPi2. I purchased a couple of these new old stock for $60. I am hoping that the RPi4 comes with 2Gb of RAM and runs 64bit Stretch in the future. I have turned my RPi-1's and RPi-2's in to little slaves hub and spoke like to the main Homeseer mothership.
Homeseer Zee2, Homeseer Standard and Homeseer Pro had no issues running on the PipoX7 using an Intel Baytrail and 2 Gb of RAM dual booting to Windows 10 32bit or Ubuntu 16.04 64 bit Linux. Many new Homeseer users are running Homeseer on the new Intel Skylake micro PCs today. (Linux or Windows 7-10).
What I am starting to see is that many Homeseer folks are running hundreds of variables, events and triggers, 5 plugins, et al on the RPi2-3. This does push a bit on the RPi as it is still a tiny computer with 1Gb of RAM.
Working on documenting next RPi project which will be posted as a blog which is using a software defined radio for downloading NOAA satellite weather maps with the POE RPi directly connected to a satellite antenna in the attic for direct downloading of weather maps (from satellite versus internet).
There is an SD radio project out there in internetlandia (you can purchase an SD USB radio now for less than $10) to be able to receive and utilize wireless sensors (and weather sensors) like or similar to the RFXCom receiver.
Relating to the above and Homeseer this makes integration of the Amazon Alex very easy. Many Homeseer folks are installing Amazon Dot's on their ceilings for an Amazon Echo omnipresence.
Relating to the Wemo a Homeseer user wrote a bash script to sync the Wemo switches with Homeseer 3 using the Java Wemo bridge in 2015. He found that the Belkin Wemo switches as fast as the ZWave switches and cheaper in Australia.
How to use Belkin Wemo devices with HomeSeer
Re Reading my post above it is kind of a splat of a bunch of stuff.
Original OP mentions
1st option would be the Alexa UPB bridge for $600. Seems WAY OVERPRICED and limited
2nd option would be some sort of software controller like CQC or HomeSeer. I guess you could buy Homeseer hardware controller as well.....I like CQC given the involvement of the company here on these forums.
added after original OP
3rd option would be the DIY offered options above.
Personally relating to the OP would be to utilize software integration for the Elk M1 / UPB and Alexa Amazon like CQC or Homeseer or a DIY solution using the RPi. It is up to the OP.
Here biased to using Homeseer as I have utilized it since the 1990's. Homeseer Elk M1 integration was provided to Homeseer 2 in the early 2000's by Randy on the Homeseer forum. Randy upgraded his plugin to Homeseer 3 as soon as Homeseer 3 was available.
Free Homeseer 2-3 UPB integration came with HS2 and was ported over to HS3 a few years back and a paid 3rd party UPB plugin is offered updated to current UPB devices. I have tested both and while the free UPB plugin works the 3rd party UPB plugin (Spuds) is much nicer and has many more features to it.
Homeseer Amazon Alexa integration to Homeseer 3 came days (literally) after release of the Amazon Alexa. There is also a free Jon00 plugin available (and a couple of scripts)
and
Homeseer 3 Zee2 is / was available for 50% off putting the price to around $65.00 USD.
RPi3 Zee2 including power supply, case, ZWave and HS3 ==> $65
Breakdown
1 - RPi 3 with case and power supply ~ $50.00 USD
2 - ZWave device ~ $40.00
You are getting the Homeseer 3 software for free with the purchase above. If you want you can repurpose the RPi3 and ZWave device to making it a ZNet device (ethernet ZWave ZNet like device) and put the HS3 OS on any Linux computer and or another RPi2-3.