Once you have the $500 Budget done you maybe want to consider to continue on that path of building your car at your taste, or sometimes you just have a bigger budget out of the gate and ask yourself what can I buy for 1000 dollars to upgrade my car. If you go with the second option I would recommend beginning with the parts in the $500 Budget and add the sway bar and suspension.
Rear Sway Bar (RSB)
An anti-roll bar (anti-sway bar, sway bar, stabilizer bar) is a part of many automobile suspensions that helps reduce the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite (left/right) wheels together through short lever arms linked by a torsion spring. A sway bar increases the suspension’s roll stiffness—its resistance to roll in turns, independent of its spring rates in the vertical direction.
There are the companies that you can choose from:
Suspension (see the complete thread Here)
Suspension upgrades can come in a number of different components and the overall aims is normally to reduce ride height, to improve the center of gravity, improve transitional weight transfer rates under acceleration, deceleration and cornering loads.
You can choose between spring / strut combo or a good set of coilovers.