Type of Mustang would be nice to know. But I can help.
1. Subframe connectors and lowering springs. Brand is your choice. Spring opinions are like a$$holes...everybody has one.
2. A set of rear UCA and
LCA. No need to spend big money. Budget set from Blue Oval Industries is fine. They are only like 169 or 179. They have poly bushings, and grease fittings. Made my notch ride WAY better on the eway and stopped my wheel hop.
3. New MOOGS front
LCA bushings and MOOG ball joints. Ppl over look this stuff because it is time consuming. I spent 235 on these same parts and ahving them pressed in. OR...you can buy a-arms from Mustangs Unlimited with new stuff in them for 189.99+shipping.
4. A good set of struts and shocks will help as well. These get pricey as heck, but any replacment brand that is cheap like KYB will be better than stock fox stuff.
5. Don't overlook simple and cheap bushings. My fox sway bar bushings and links were wasted. I bought new poly bushings (size specific to bar) and I actually bought 94-04 sway bar links and bushings. The 94-04 links are thicker than the fox links and might help a little.
6. Poly rack bushings. I installed mine. The rack did not rock back and forth anymore. With the rubber ones, it moved about 1/8-1/4 inch. Imagine how it moves over bumps, cracks, and pot holes.
7. Get new MOOGS inner and outer tie rods. I installed them..wow, way tighter steering and handling. Car didn't wander anymore.
8. I would suggest a steering shaft, but 180-200_ dollars eats up most of your budget right there.
These are mostly upgraded stock parts. I didn't recommend anything fancy because you don't have much to play with. Sometimes you don't need much to have some fun!