If it were me, I'd be looking for an early poverty spec 2.5 (non-air con with as little to go wrong as possible). Stories of engine issues on them are rare these days (the supposition being that all of the ones which were going to go already have, but they were always the one with fewest failure stories), cable throttle, no door airbags, lighter than any other post-air cooled pork, etc... More likely to be collectible in the long term too, and can be bought at almost break up value at the moment (reasonable sub £4k, runners sub 3 and basket cases occasionally sub 2 : which perhaps explains why used early Boxster parts aren't exactly rare or expensive on eBay).
Budget for some bills though - lots of tired ones out there.