You have a ton of options.
Firstly the Marantz 6003 can be had for not too much more than the 5003 right now. Also, I wouldn't buy a used receiver unless it was a store model from an authorized dealer so you still get warranty coverage.
Secondly, I think your speakers are probably more power hungry than a receiver can generally provide (which speakers did you get?). So you should be considering an external amplifier and using a receiver that has pre-outs.
The Marantz 5/6003 and Pioneer Elites all have pre-outs. Also, I would look at a higher end Onkyo like the
TX-SR876 if you're not too interested in an external amplifier. If you are going to consider a separate amplifier, than the Yamaha RX-V765 has some really nice processing capabilities and can be had for around $450 if you look around long enough.