Budgeting correctly is far more important then anything else. If you only have money to buy RCA's next then just buy them and wait on wire and such. Just pull out the system set it in your room and get what you need over time. Once you have everything, put it all back in. Check in here or other forums and see what the general thought is on the new install. Take pictures for grounds and battery connections and everything. That should put you in a position to save money not having to replace anything.
Make sure that when you redo the wiring you get good terminals and crimp or solder them on the best that your ability allows.