The GSXR will have the most power on top, the GSX will be more oriented to the mid range, like the 600, just more of it everywhere.
For best results, the carbs and ignition box that goes with the motor you choose would be best.
IIRC, there was a 750 Kat conversion done by a member here several years back. I'm thinking he didn't use the Kat ignition box, but I'm not certain.
Your best deals are going to be from private parties. Salvage yards usually want quite a bit for one, especially a GSXR unit. You may also be able to buy a complete bike in cosmetically challenged form and take everything you want off, and part out the rest to help recoup your investment - or if you're really lucky, turn a profit. Assuming you have the space.