Just an FYI, while Surveillent has the lowest prices around, you are on your own if anything breaks during the warranty period.
I had a keypad recently go bonkers and they told me that it would be less expensive for me to buy a brand new one from them rather than shipping the unit to them. I would have to pay for shipping 3 times - ship to Surveillent, pay to have Surveillent ship to their distributor and pay for shipping back from Surveillent to me. When I asked them to just send me a copy of the receipt from their distributor so I could deal directly with Elk, I didn't hear anything back.
On another note, retrofitting wired sensors is a ROYAL PITA. I went with mostly wireless through the Micra ION sensors, but left a few doors on wired because I figured I had easy access through the attic and didn't want to spend $30 on another sensor. I am now kicking myself for not going totally wireless on the door sensors. In some cases, it has taken nearly an entire day for me to drill up to the attic and fish and tack a wire from my panel in the garage to the right spot - FOR A SINGLE SENSOR. I don't necessarily work slow, but this process has a ton of potential issues and it has really discouraged me.
Speaking of discouragement, I still have 4 wired motions and 4 wired glassbreaks to install in our 4 bedrooms. That is a lot of work!
I hardwired my whole system, in a finished house. It took a while, but when beginning the project I told myself 1 or 2 sensors a day. If it took me a whole day to do the 1 or 2, so be it. If I got them done quickly, I wouldnt push for more. Eventually I had everything done: 4 GBs, 4 motions, 10 windows, 3 doors, 2 non-alarm plungers, 3 keypads, 5 smokes, 2 heats, 3 COs (along with the cell communicator).
To me the time was well worth it....Ill never worry about batteries or "strange goings on" from wireless.
As far as WHERE to buy, I bought most of my components off ebay. The panel itself I would stick with an authorized reseller, even if it costs you 100 or 200 more. The keypads, thats up to you. The sensors I wouldnt hesitate to just get from wherever (I dont like Surveillant just because of their horribly over-priced shipping, but there are plenty of other good alarm suppliers on ebay)