I'm looking at a 1999 to 2005 996 c4 or c4s my question is mileage.
I have read/heard that high mileage is better then low mileage do to half shaft bearings / seals.
found a 2001 with 55K and a 2004 with 90K price about the same. I could replace the engine with a rebuilt one on the 2004.

This would be my 2nd Porsche in my life, the first was a air cooled speedster. My son has a 80's Targa.

This will replace my Z3M which I crashed into a truck. (not my fault) he made a left hand turn in font of me at 55 couldn't stop.

 

2004 with 90k is only 10k per year, sounds like it was a DD, which one would think was taken care of. Get a PPI. At 90k on it you'll only get about another 160k or so out of the engine.
On a serious note, do a side by side comparison, engine size, tire size, features, overall condition, what if anything is going to need to be fixed right off? etc and go from there. Look at the interior and the condition. Ask the owners for service records, call the places where they were serviced.
Now if it were me, I'd steer towards the 2004, but that's just me.

 

Instead of "replace the engine with a rebuilt one", why not take the extra $$ and look for an extra-nice 40K-55K mile 2002-2004?

Personally, I like the MKII's (2002-2004) much better than the MKI's (1999-2001), and while I would definitely shy away from some 996 that was never driven, an '04 with 45K miles on it is just about perfect usage-wise...

 

and to think i though you were talking about one of these

 

Personally, I would go for a C4S.

 

I'm not buying this whole "Low mileage is bad" thing. Aside from the IMS, which can and should be replaced with an improved version, what other significant issues are associated with low mileage vehicles? I wouldn't avoid a low mileage 996 if the maintenance records and PPI are in order. I would however, be sure to address the IMS as soon as possible, regardless of mileage.

 

A few possible issues come to mind:

- If a vehicle is left to sit for extended period of time every seal will dry, crack and leak.

- Condensation in the engine can lead to corrosion.

- Time based deterioration of oils and coolants can cause problems (too many owners only look at the mileage requirement)

 

A low mileage car doesn't necessarily indicate a car that has sat unattended for extended periods. That's why I said check the maintenance records and get a PPI.