On my old beater celeberty the exhaust was gone after the cat as well. What I did was cut the cat off with a sawzall and got a universal muffler that just barely slipped over the pipe where that cat used to be, clamped it with an exhaust clamp and used some mechanics wire to hold it up. Looked like $hit but it worked
For the tires just go get a used set from a bone yard, they will probobly sell you a set of used 14" tires for <50 bucks as used tires aren't worth much. Get the caliper from the junkyard if you can too, they may be hard to find one in better shape than yours but most gm models should be similar so you should be able to find some car that has a decent caliper on it.
The O2 sensor you should buy new, they are a bit expensive but they will kill your gas mileage if thier bad so it will pay for itself quickly. Just let the shocks go, for a winter beater it shouldn't really matter unless your trying to play jr. drifter with it
My $.02