New Radiators are fairly cheap, you can even go as far as getting a junk yard one if you are looking to cut costs a bit. The "full tune-up" I would be a little leary about, mainly that usually consists of changing the spark plugs and going over the diagnostics. Pretty much you can get your own plugs and replace them on your own and save a ton of cash.
With regards to your axle noise, is a POPPING noise when you make a turn in one direction or is it the same for both? That would generally indicate a hub going bad if it pops in one direction over the other. Half shafts for our car are fairly cheap and somewhat easy to replace. I picked up a set from Rock Auto by CarDone1, they ran me $105 with shipping.
As for your brakes, the same applies here, you can go on eBay or hit up TDC for a set of his rotors, if you are still running factory discs on a 98, it is time for sure to replace them and the pads. A good set of rotors and pads will run in the $200-$250 range and that usually includes shipping. And there are enough tutorials or how to's here to allow you to do the job yourself.
The exhaust issue you mentioned, do you have a hole in your current resonator? Or is it just sounding like crap? Once again this is one of those things where you can hit up a junk yard and find a newer model GT and take its exhaust/catback to replace your old stuff. Or you can get the SLP catback setup for around $450-500.
Just some thoughts on your questions![]()