I have 993 Targa and can offer the following observations (all from experience of my car only - so may not be representative of all Targas!).
- The seals tend to shrink and leak a little as they get old and will probably need doing at some point if they haven't already
- The front drain tubes seem to rot away, so you can end up get some very slight leaks into the cabin (particularly when parked on slopes, I find). Check the carpets and sound proofing for damp/water
- You can get some wind noise (usually a whistle at certain speeds) if the roof needs re-aligning. It will never be as quiet as a coupe roof
- The glass is very expensive to replace (and seemingly difficult to source) should you ever need to
- The blind mechanism is one of the most ridiculously complicated affairs you could imagine. Not Porsche's finest day...
All negative, right?! Actually, no. I have a very good Indy who has done a lot of work on my roof, synchronising the cables, replacing the seals, re-aligning the roof sections, etc. Not too expensive but takes a while to get right and can be frustrating. I now really like having the Targa -it gives you a great sense of space and I have the roof open a lot more than you would imagine. Wind noise at motorways speeds is not an issue. In fact, in a poorly adjusted roof that whistles, just opening the deflector will actually reduce wind noise.
They are certainly more trouble than a tin top, because there's obviously more to go wrong, so you have to decide what it is you really want. I enjoy it, but if you're going down this route make sure that the a/c works and that the blind retracts/deploys without issue. An unshaded, uncooled greenhouse is not the best place to be when the weather gets warm...
By the way, plenty of people will tell you about how the Targa is compromised, not as pure or focused, has less rigidity, etc. In the real world this makes no difference. On track? Very little difference that I've seen (unless you're actually building a race car - and like the old joke says, if that's where you're trying to get to I wouldn't start from here).
Cheers/John