Well if your car's
suspension is stock, I would do the Springs, and would do shocks and struts.
Your stock shocks/struts would be fine for a while, but will need to be replaced in the near future. The lowering would really be harsh on them.
I did my springs and shocks/struts first, also put in some gt500 strut mounts. Springs(
eibach) set be back about $250, my D-specs were $550 and the GT500 strut mounts were $60.
I don't know if you have ever seen some cheaply lowered cars, like civics, or acuras. When they go over bumps/waves in the roades the car just bounces like it is strictly on springs, that could be a very real possibility that could happen if you only did lowering springs, especially if your car is a few years old.
That would be my first "$800," but I guess if you did that, you may/ or may not need a
panhard. Which would set you over your budget, but you could go with a cheaper shock/strut setup than the D-specs.