[kk2002s]

A PPI would certainly give you a better picture
I only found one other 99 w/104k miles for $5,999 (Auto trader)

As it's a third car you have the luxury to let it sit to figure out what is wrong and repair if something breaks
If this is not fun money to spend then you need to do the work your self. Labor to fix will kill your wallet.
That also means you need a place to fix it, tools and the patients to learn, make mistakes and do the work yourself.

It will be those little odds and end breaking/wearing out that will drive you crazy and questioning it's dependability. It only take one little thing that shuts you down somewhere away from home.
For me it was a $10 Fuel Pump Relay, easy, cheap fix but it took time to figure it out, internet the part in.
It's not a Miata

The dead motor problem, like others said, you could part out, etc to recoup a good share of your money

You could also get a GRIN maker that just starts and runs
14 months into ownership (and a 'S...' load of GRINS) and logically buying an older Boxster still doesn't make much sense
Now Emotionally, that is a whole different story - Hell ya it makes sense