Welcome to the forum. I have had my 2001 Box base for three years now and the 2000 Box base has the same engine. My feeling is that the lower the miles the better. I bought mine with 30k miles and now has 40k miles. I believe that the lower the mileage the less the wear on the accessories.
However I notice that some accessories have been replaced. I have had installed the IMS Guardian. Some would say that the lower miles on the engine has the greater chance of IMS failure.
120 k miles is pretty high but some owners here have 200k miles on theirs. Get a good PPI done if you consider this one. Verify the year of manuf, mileage and work done. Receipts? Do not be in a hurry to buy one. Try as many as possible to know what normal noises are and what are not. Before u start the engine make sure all warning lites light up particularly the CEL one.
If it needs work be prepared to wrench on it or pay somebody to do it. It is a fun car.
Sometimes I wish it had the occasional back seats like the 911 but then it would not be a Boxster. Enjoy your search and try and be patient.
I have been very fortunate in that I have not spent too many $$$ on mine in the last 3 years. CV joints, Ims guardian, tires etc . Cheers, Guy
