General rule of thumb is buy the newest low mileage Boxster you can find...
Having said that, take a look at Mike Focke's excellent guide to buying Boxsters.
Part 1
http://mike.focke.googlepages.com/buyingorsellinga1997-2004boxster
Part 2
http://mike.focke.googlepages.com/buyingcontinued
you'll want to know the service history of the car before you can really figure out what will need done both short, intermediate and long term. A new clutch is going to run $1,500 to $2,000 to start. Airbags... figure $700- $1500 each min depending on if they are door, steering wheel or dash. Curious as to why it needs airbags as you didn't mention any accident/damage
.
Yearly maintenance (assuming all records up to date) will be $500-$1000 and Boxsters love tires - 20K on fronts, 10-15K on rears depending on how you drive at $100-$200 per tire depending on your preferences.
This should be enough to help you get started. Good luck and I'd definitely get a PPI (private party inspection). Yes, it'll be $2-3-500 but $ well worth spending - 'friend' or not. The saying goes "nothing can ruin a friendship faster than trying to do business w/ friends".
Good luck
