prob with using the rca convertors is that factory headunits are programmed to roll the bass off at higher volumes to protect factory speakers. The only way around this is to fit something like an MTX RE-q3 or 5 unit that compensates for this, and gives you more adjustment and a remote turn on output too. These will cost, so its always better to go for an aftermarket headunit. Can you look for a second hand / last season model to save money? It would be well worth starting with a good unit to then build upon at a later stage.
I would happily assist...but your a bit far from derby!