If you want cool and retro, go for a Mk2 or a caddy. If you want something less retro and possibly more modern comfort, go for a Mk3.
If you do want a Mk3, but don't want more than a 1.6, look for the late R reg 1.6 GL which got a new 1.6 100bhp engine (developed ready for the Mk4). There's a few around, but it may take some time to find. Should cost about a grand though.
I didn't think the early caddies were SDI?
Are you talking about Mk1 pick up style caddies?
Anyway, if you aren't bothered about passengers that's fine, but what's the point in putting audio in the load bay? Sure you could put a sub and amp in there, but there'd still be a bulkhead and glass between it and you and you would have to make the load bay water tight so your gear didn't get wet.