Hello mr.borrie,
The universal OBD2 diagnostic tools won't give you any deeper details on a Porsche. Especially on the engine (for example camshaft deviation – which would be important in that case). Also you can't always be shure to get a correct error reading. Which makes it useless in the end. There are some after market products on the market like Durametric. But the good ones are expensive.
The Porsche workshop manuals can't be officially bought from Porsche. But the internet gives a lot of (sometimes not that legal) possibilities.
The engine problems are well known. Also if you do an IMS replacement, the engine can fail in other parts. Some engines are very short lived, some run forever. Hard to give a recommendation. If you can live with the idea of a failing engine or the idea to loose a lot of money, than go for a Boxster. If you are on budget and it would hurt to throw money in a car i would say look for something else. There are a lot of cars out there that are fun. It must not be a Porsche.
And just don't think of failing engine parts. A car that is about 20 years old needs also brakes, suspension and other parts. Especially if it's a Porsche that wants to be driven like a sports car.
Would i buy a used Boxster today? I'm not shure – think with the knowlegde of today i would go for something different. I bought mine new in 2003. So i know the history, i know what has been done to the car and i also know what can happen in worst case.
Regards, Markus