See my previous post.

Start with pawn shops, go with major brands like Alpine, Kenwood. etc

Try to get a H/U with at least a 4 volt pre out.

Always test used audio equipment before you buy!

With that budget you could get a starter Alpine H/U, JL audio slasher series 4-5 channel amp, perhaps some used Alpine 6.5's and maybe even a sub box with some kickers to round it out.

Be patient, use upgraded wiring and cables.

Have fun.