Rule #1: There are no cheap Porsche's.
Boxster's are cheap to buy because they are expensive to maintain and the engines have questionable long term reliability. Very few Boxster's make it over 200K miles before the engine fails. Used engines are available and can be purchased and installed for $5K so its not a catastrophic event if you plan accordingly.
You should expect to spend $2K in maintenance/repairs per year on average. Some years will be less and some years will be more. And if it ends up being less, consider yourself lucky. And that amount applies to well-maintained cars. Ignore things and the costs will go up.
Just don't freak out the first time that a shop hands you a $1,200 bill. Or the second time. Or the third time. Think long term and you'll see that these cars can be a very good value.
With that being said, there isn't a better car to buy for less than $10K and even if you have to put $2K a year into it for 10 years, you've still only spent $20K for 10 years of total driving enjoyment.