I think your challenges are reasonable.
The IO Gear keyboard I'm using is 2.4 GHz RF so it has no problem functioning in the adjoining projector HT room while the PC sits in the TV room. I haven't tested it's distance limitations yet as I've had no need to. However, I've set up the web interface on my wife's iPad just for fun and it works great. That gives me control as far as my wifi flies. Useful if you have a multi-zone music system or additional TVs hooked to it.
Ideally, if your 2nd location is within 25 - 30' cabling distance and it can be done, a physical HDMI connection to the second location would be preferred for the least amount of HD hassles.
However, you could also purchase a small dedicated streaming device that can access the main library of the HTPC while also providing Internet based content as well. There are lots of solutions out there for under $200.
For a small footprint computer around 600 that is still useful for other things than TV, consider a Mac Mini. About $600 when I looked at them.
An AppleTV streams content pretty nicely for about $150 although you may or may not have to jailbreak it to get it to run something that will access the main library on the main HTPC in the other floor. There are a lot of AppleTVs selling right now as the cheapest solution to run ICEFilms plugins, granting you simple & instant but (illegal depending on where you live) access to virtually every reasonably well known TV show/movie ever made in the last 30 or more years.
Just some initial thoughts but I've run out of the section of this conference call that I'm not deeply involved in. I have to actually pay attention now.
