HEIL EKONA
Ahem.
First off, great choice in spec, that would pretty much be my exact pick too IMS you can't check (realistically) so maybe fit an IMS Guardian if you're particularly paranoid, but honestly I don't think I'd bother. Bore scoring/d-chunk is a bigger issue, in terms of inspection then I'd speak to Mike at Sports & Classic in the first instance as he's the go-to guy for stuff like that. There are others though, depends where you are in the country.
996 will be cheaper to run, although you'll probably need to replace a few more age-related bits. Both cars will cost roughly the same as the Zed over the course of ownership (my 997.2 cost me no more than my 350). www.planet-9.com is the place to go for info on the 987 cars, US-based but the info is staggering. Nice people too.
In terms of the drive, they'll wee all over a 350 or 370 from a rather large height. Steering control, chassis feedback, driving position, brake feel... It's all a cut above. Engine doesn't sound like an anaemic hamster either! Wonderful, wonderful cars.
If it were me, I'd take a 996 over a 987. Mostly because rear-engined cars are simply hysterical when you get used to the weight transfer, and it takes a bit longer to learn too so you always feel like the car has something to offer you. That said, the 996 styling (inside and out) isn't for everyone, which is putting it nicely Each to their own, both cars immense, just make you you're happy with the inspection before you buy.