Buying 986 with m97 engine replacement
 

Saw a listing for a 2003 S with a replacement m97 due to earlier IMS failure; when searching I found a Jake Raby post saying the m97 bearing has no spotted failures except for tracking (since they redesigned it to last under street driving instead of racing, like the earlier). My local mechanic says not to trust it despite that, that the m97 2005-8 was designed to avoid in warranty failures and that is why eventually the IMS got removed; and that anyone, if they had their m97 engine apart, would replace the bearing anyway even if never "tracked".

In any case, in general is a replacement m97 engine in a 986 like the listing above, a good or neutral or bad sign?

I don't like bastards,but if it was done properly & the price is right,(it's not an upgrade in my opinion) the problem is serviceing many mechanics won't know what to do & where parts are located. Much easier to buy a original car & replace the IMSB with a new one of your chose from L.N.

They're both right and maybe a little wrong. The Gen III IMS bearing (mid 2005-2008) has approx a 1% chance of failing. To replace it, you need to totally disassemble the engine halves to do it properly (there are some procedures where they're drilled out). But for the most part, the GIII IMS is reliable. Your mechanic is right from the standpoint that it is a bearing and ALL bearings eventually fail but this version should last 200K+ mi. There's a lot of speculation why IMSs fail, but tracking your car at DEs is generally not thought to be a reason IMS fail. Low mileage cars that sit for long periods of time have a high rate of failure. The other item that seems to correlate to failure is oil change intervals. You're ideally looking for changes every 5-8K mi. Anything longer could be an issue as the IMS is bathing in dirty oil. I've given a very high level answer, if you want more detail there is plenty of info on IMS here in archives.

I would be more concerned about the electronics than IMS. The M97 would be using a different ECU and would have different features / functions than a 2003 had. You'll want to be sure all the electronics are sorted. Can be done, but would be a big job.

Then I am right to think that getting a 2005-8 base (or a 2005-6 S) "solves" the "did you replace your IMS" questioning I have to do to each 986 that is advertised? My local technician said you don't want to be the first guy to find out when these III bearings fail.

Not exactly...

Porsche implemented the GIII IMS during the 2005 MY. So a 2005 could have a GII or GIII IMS. Supposedly the changeover occurred in mid-Mar 2005 but no way to know for sure, esp if the build date is close to Mar. Porsche did not keep track of which type of engine went into which cars. Porsche had a batch of engines in a warehouse and grabbed an engine as needed. Supposedly they'd grab a GII out of inventory before going to a GIII, but no guarantees they took them in order. Only way to be certain is if you have an early or late build date but best bet is to remove the trans and look. GIII is larger. Or you buy a 2006-2008 for a GIII or a 2001-2004 for the GII (2000 MY was a transition from the GI IMS to GII). An early 2005 with the GII IMS will be serviceable w/o splitting the engine and will have the design that's prone to failure 10% of the time.