this is the very reason i bought a sampler. it is time consuming and quite a head scratcher to get all of those synth sounds into a sampler (especially modulated sounds) but is very rewarding. sometimes i have to sample whole chords into one note to save memory, but that actually just makes things easier. a sampler will give you ton of flexibility as far as playing and song writing go. plus now i only carry 2 keyboards to gigs, my sampler and a microkorg.
if you want to go with somehting older look for something with at least 8 megs of memory.
emu emax II with expanded memory (what i use)
ensoniq asr 10
ensoniq eps 16+
kurzweil k2000 or k2500
newer (most are racks)
emu esi series (i use an esi4000)
emu e series
akai s series (s3000)
yamaha a series (a3000, a4000)
roland v synth (me pinning my gear lust onto you
one thing - find something you can put a hard drive in.
it wont be easy, but it will make you a thousand times more creative.
i would definitely look at the kurzweil stuff, simply because it has the rompler potential to replace your alesis q thing without having to sample.
Brains can be used like a "stress ball," but only once.