DIY Rebuilding an Automatic?

[Canyonero]

My Trooper may be out of commission for a little while yet, and I'm eying another Trooper for sale around here. It's the same generation as mine (88-91) but with an auto, truck has 180,000 miles (mine was running good at 163k). The guy says it runs great, but the tranny is slipping. He has not investigated the slippage at all other than checking the fluid level which he said "looked fine". So, do I buy it and hope all it needs is a filter change? Or if I buy it and it ends up needing rebuilt, is that something I will be able to do myself (after my other trooper is going again of course). I've never owned an auto before nor done more than a filter change - what does slippage usually indicate?

The tranny code is 30-80E A340H.

Thanks.