Yes the A/C and PS can be installed on an SR swapped car. It's a lot of work sourcing all the parts and can get a bit expensive. It's no cake walk. That's why so few of them have it. It certainly can be done though.
Swapped 240's show up for sale occasionally with A/C and PS. That is probably going to be a much better idea. I believe one of our Mods have a S13 coupe for sale with the Silvia front end conversion and an SR20 engine and PS. Not sure about A/C though.
The thread is closed, maybe he's keeping it?
http://www.sr20-forum.com/cars/24965-fs-s13-240sx-sr-swap-silvia-front-heavily-modified.html
Think long and hard about which body style you want (coupe or hatch). I used to like the coupes and wouldn't even consider a hatch. Now after seeing them in person many many times I think the coupes with the Silvia fascia look awkward, and the hatches look amazing.
Your results may vary.