Honestly, there is no need for rear fill in such a small environment. It will have no benefits for your listening experience up front, and a well-installed front stage will be more than enough for the occasional rear passenger.
Trust me, you'll like it better.
I would save up a little more before you start looking for stuff, because your options open up drastically when you get into the $200 range for components. However, if you must go with the budget you have now, Phoenix Gold RSD components can be had pretty cheap and I have seen some good reviews on them.
A PG RSD sub would be nice too. Very nice woofer for around $100. Amps are really the hardest part of the system to find cheap. You can go with the Profiles to match if necessary, but you'll get better quality from some of the used stuff in the classifieds.
Bottom Line: Bringing it all in within your budget is possible, but not easy. A little more cash will open up a lot mor possibilities for you.