hi, welcome!
you actually only need the SID chips and if they work that's a great start.
if you decide for the MB6582 you can also use the power supply and the power switch for example.
all parts besides sids are easily available and no problem getting them.
here's some info about the MB6582:
http://www.midibox.org/dokuwiki/doku.php?id=wilba_mb_6582
unless you have a sammichSID this is the easiest (albeit large) solution in terms of wiring.
and if you think you might like it so much that you probably want more voices, just do it. you can build it complete and just stuff the one stereo part and add the other ICs later.
here are complete base board kits (and boards) (pos 70 and 72) available, encoders and other individual parts like preprogrammed PICs too:
http://www.avishowtech.com/buy.html