the frame is a 2009 Giant TCR Alliance 1, 105 front derailleur, crank shimano SG-X 50F 34F (think it's the stock 105 crank and chain), pedals and seat post clamp stock, and a used saddle. I need a lot of parts including a rear derailleur, cassette, shifters, and brakes what parts can I mismatch within the shimano & sram brands?

also I've been thinking about setting it up as a single speed because it will save a lot of cash and weight. I need brakes (brakes, cables, levers etc), seatpost, headset, stem, handlebars and a fork, (i think I have a wheel or two that are ok)

IIRC the frame uses vertical drop outs so going single speed will likely require a work around to allow for chain tensioning adjustments, like a tensioner or a eccentric rear hub/BB. Not real cheap solutions.

I STRONGLY suggest that you keep within the Shimano family with the drivetrain parts. There was a recent thread here about some one spent over $500 trying to upgrade their drivetrain only to find that the performance sucked enough to spend another chunk of $ to make it work well. He ended up with an all Shimano system.

Where mismatches work best are in the brake calipers, wheels, pedals, brake levers (if you're using bar ends or frame mounted levers). While many will claim that off brand cranks and rings work well we see enough trouble with many to not be able to predict the end shifting performance well enough to suggest going that way.

One possibility is to buy a used bike that already has the right parts on it and transfer the needed stuff then sell off the remainder. That creaking you hear is Pandora's Box opening... Andy.