A Mk 1 Wal in good nick sub 2.5k would be a decent investment and one you will unlikely to loose money on as long as you kept it in good order. Old Fenders (pre CBS) will always be good investments if original, but that's the problem now days - there are so many fake or bastardised examples that I would be wary of anything being sold under the going market value which is in excess of 5k. But if you do get an all original example that plays/sounds good it will be a gilt edge investment.
What was said in other replies around proven consistent designs being the best bet for 'investment' reasons. Ltd editions can be a little hit and miss - I lucked out getting hold of a couple of Rickenbacker Chris Squire's at reasonable prices as these seem to be in demand and don't come up too often - I think these are the only basses I have bought with investment in mind, although the real reason was me being such a Yes nut when I was at school in the 70's I wanted them for that reason most. Other than that I can't think of a Ltd edition bass that has that 'demand' factor.
Bear in mind making an investment typically means holding onto the bass for a good few years. I've had my '62 P Bass 36 years, which my Dad paid 100 for, and my Stingray for 31 years, although this did cost me 450 new.....The thing is, they have both paid for themselves hundreds of times over (as have most my other basses) through gigging and recording.