Why?
Because it's an old Boxster. Brilliant cars, but it remains a Porsche. When they break, they cost big money. When it needs a service, it costs more than your 1 Series. It will cost a lot more in fuel. It will cost more in tyres - it's a Porsche, the fact its now cheap doesn't stop it being a Porsche and the running costs on a car like this go UP as it gets old, not go down.
There is, of course, nothing wrong with buying an old Porsche but its the sort of thing you do when you've got plenty of money swilling around and you don't mind sinking it into keeping an old Porsche on the road. Generally this situation is not one that leads you to having to take a crappy finance deal from a used car dealer (And it will be a crappy finance deal, I would be amazed if a finance deal from an independent trader is at all competive, all he's doing is offering somebody elses finance).
I just don't think this is the right car for you to buy, at all. Everything seems wrong here - the way its being funded highlights that this is probably a stretch, quite a big one, but you dont realise it is because you've failed to take into account the costs involved with such a car. You seem to think because it has a 'warranty' every thing will be fine. Yea, right! It's nearly 10 years old, the warranty company wont pay out for a thing. Warranties from independant traders are worthless.
Getting an Inspection done after you buy the car is completely wrong. You do it before you buy it, why would you do it any other way? Get it inspected THEN decide if you want to buy it, dont buy it, get inspected and then decide to return it! What if he's already sold your PX!? I'd be amazed if he even sells it to you, the fact you want it in writing that you can return the car in 2 weeks time would make me, as a dealer, tell you to walk anyway, because it just sounds like you'll probably drive around in it for 2 weeks then chop it back in.
If you sign the finance on his premesis I'm pretty sure there is no cooling off period anyway. Otherwise everyone can get a free hire car for 2 weeks by financing an M5 and then returning it!
stop thinking that because a Porsche is £10k its just like a Focus and buy something more appropriate. If you want to upgrade a diesel 1 Series to a decent sports car the Mk3 MX5 is a fanatstic car, it'll be nearly new, it'll be reliable and it will be a fantastic entry step into the world of sports cars.
The time is not, IMHO, right for you to buy a Boxster. Leave the Boxster for the future when it's quite easily affordable and when you dont need to faff around.
Seriously - think about this. Your last car purchase was by your own admission completely epic fail and you've admitted you regret it on here. Dont make the same mistake again.