I work as a Porsche mechanic so here we go.
>1999-2004 996
There's a reason they're so cheap relative to other Porsches, and it's because they have become the daily-driver amongst people who like Porsche in general. Don't expect a 996 to be too well-looked after, most of the 996/986s that I've worked on tend to be in pretty awful condition compared to the 9x7s and 9x1s.
996s are very easy to work on considering they're a rear-mounted engine with RWD.
Base or S 996s are fairly reliable assuming maintenance is kept up, you'll see average things go out like your bump stops, top hat mounts if you have a 4 or 4S, water pump occasionally, transmission mount or oil separator. Check service history to see if any of these repairs have been done, if they have, fuck yeah.
IMS is also a thing on 9x6 and gen.1 9x7 engines, but if you were looking at porsches, you most likely would have came across what its about.
Watch out for oil separators. If the car's gone past 100k kms without a new oil separator, expect mad money as its an engine-out job on a 911. On a cayman/boxster, considerably easier to replace, as mid-engine porsches are literally a smaller 911 engine put in the middle and turned 180 with the transmission following suit.
>Cayman S
One of my favorite Porsches actually, neutral handling, great cargo space considering the size -- a hard-top boxster without the billy joel syndrome.
Same sort of deal with maintenance of the 996 barring top hats, never seen one go in a cayman/boxster. Oil separators arent as bad either as you dont have to pull the engine out. The transmission mount in a cayman would be the engine mount, as per the whole "same engine but 180 degrees" deal.
Try to find a 2008 onwards, as the gen 2s have a new engine and if you didnt want manual, the PDK transmission is actually amazing to drive. New engine means no need to have irrational fears of your IMS destroying everything you love around you.
In short, fuck 911s caymans are life.