Washburn XB600's are great basses! Not that common second hand but are usually available for around £175-£200.
Yamaha RBX375's and the older RBX775's can be got for around £200 also. BB405's and BB425's may be found for just over £200 but, being passive, probably won't satisfy your itch for metal tones.
Re: amp
Probably best to go for a multi-fx or preamp pedal with a headphone and aux-in socket. This way you can plug your iPhone in and jam silently. Also, a lot cheaper than buying an amp. Zoom make several good headphone amps, have a look!
I'd guess that those little combos you've listed won't be very effective at reproducing those lovely low B's.