I had similar concerns when I started my project - I don't want to be in my house more than about 3 years... but I went with the M1G anyway... When it really came down to it, I figured what I might do is buy an EZ8 system when I go to move out so I leave only the absolute most basic stuff (and only costs a few hundred $ tops, keypads should fit the same holes, etc)... but take all the goodies with me. The bulk of what I've spent so far is in expanders for lighting, extra zones, sprinkler control, etc. However - when I go to move out, I'll be taking the UPB and probably the M1G (unless there's something way better) with me along with all the accessories and leaving only a basic functioning alarm system that looks no different than what they saw when they walked the house with the realtor. May cost a couple more bucks than an even more basic system, but I'm hoping to be able to use the same keypads so I don't even have to patch holes or anything - and can still have everything nicely flush mounted for now. They won't know there's fewer rules and no voice and no ethernet - but that kind of stuff usually scares a potential buyer anyway unless they're tech-friendly.
As far as smokes and motions - the elk should work with any of them really - they're pretty standard from what I can see... Smokes - I've read that any 4-wire will work; but only specific 2-wire ones are compatible. Those should be listed in the manual, but if you have trouble one of us can list off the specific GE models that are compatible. I just don't have a manual handy.
As far as the software - no, you only need to connect long enough to transmit changes. You can even work off-line, make your changes, then connect and synchronize everything. That's my big selling point - I'd rather not have a PC dedicated to my system - I'm more of a purist. I want my alarm and basic automation to be flawless - always on, never touched except when working on something specific to that. Same reason I use a Tivo for my shows - I don't want it to ever not work as expected. PC's are great for add-on fun stuff, but they always get corrupted, crashed hard-drives, powered off because of power outage, etc... That's a matter many people here are divided on - just depends on where the priorities on in your mind.
I want to know that the back-door sounds will always sound - I don't want to miss it because the PC is frozen and not hear that my 2-yr old nephew snuck out and is now by the pool - and I don't want to come home to a dark porch because some software glitch caused the lights to not come on... or that the doorbell isn't working because of some glitch... And, I don't want to spend a whole lot of time on all this... I could go on for a while.