At least three problems...
1. Salvage title will at best bring the cars value up to 6k with a running motor and no other issues
2. Even if you know exactly what it will cost to get the motor running, you have no idea what other issues the car has because you cannot test drive it. And virtually all cars below 10k have multiple issues not disclosed.
3. By the time you are done you will have spent more money than a quality Porsche at a higher price.
Advice.
Note the Boxsters for sale. Majority of them have been for sale for multiple months, mostly due to unrealistic sellers and detachment issues. BS with the owners and you maybe able to get a call back a month or two later or keep their numbers so you can call them back. Buy your Boxster in the fall and winter, it is the cheapest time of the year to buy them. If you actually find one for cheap jump on it as it will not last long but at the same time, look it over well for potential major issues.
I bought this one owner car for well below 4k and sold the top for even better savings. Its even got new Michelin pilots. It is my 5th boxster and probably one of the better ones I've owned. I have not paid more than 7k for any of my boxsters yet.