I'm not sure if you can really say there are any particular serial numbers to stay away from as far as a particular problem. I'm sure over the years, some small adjustments have been made on machining but I don't know of anything major to point out. Of course, small adjustments could possibly have a big impact on overall reliability. The early P-11s had a 10 lb. trigger and the newer ones have a 8.5 lb. trigger. The 10 lb. ones were a good while ago as the P-11 has been around since 1995 or 96? With some of the early P-11s a stainless slide was an option but now the choices are blue, parkerized & chrome. The latest serial numbers are 112XXX or maybe 113XXX. Mine is 78XXX and I bought it about 2.5 years ago.
The P-32 has been around approx. 2.5 years, maybe a little more. The latest serial numbers on them are now up to 60XXX. They did upgrade the recoil springs awhile back, but no big deal, KT sells them for .50 cents.
I bought my first P-32 2 years ago, it was 8XXX and bought another in 5/00 & it was 10XXX.
I guess newer is better if you're buying used but they're not expensive guns so buy new. Customer service is top notch if they need attention and they are honoring the warranty on used guns if you buy used.