Ok, the most important thing with these is a Pre Purchase Inspection. Ideally with an independent garage, followed by Porsche themselves. £200 - £250 is the going rate and will give you a very good idea of what you are buying.
A C4S is a bit of a bargain in today's market, a 911 with pretty good performance with far better looks than the poverty spec ones. Deprecation is no longer too much of an issue with them any more.
Budget 2k a year for maintenance.
Look at lots.
Then go and look at lots more.
They should be in very good condition, however some peoples "mint" is different from another's.
The trick is to get a mechanically sound one along with the interior you want.
They will have all had some kind of paint somewhere along the line.
Expect a fair few owners, they change hands around every two years.
The Tips are not bad although the ones I drove were Turbo models and have a different box. Try both transmissions.
The NA's do suffer from major problems, failure rate is around 5%.
Try to get one with the spec you want - Retro fitting and parts a very expensive. This will massively limit your choices by the time you come up with a shopping list. If it falls in the desirable category (Black with Black interior) you may have to stump up a few more dollars.
FSH is a must. It doesn't have to be Porsche all the way, a specialist is fine just make sure its not some back street garage as to work on them requires a lot of knowledge.
My advise would be to find a couple of locals and take them for a drive, you hate it. Whilst it has the mod cons its very much so a raw car under it all. To an extent, they rattle, are noisy and you feel every bump in the road.
Once you have driven a couple then start to look as as many as possible, even ones over and under budget. It will give you a really good idea as to the variety of condition out there and will give you a good indication as to what you should be spending rather than what you want to spend. Pick a lemon and you could end up with a 5k bill straight out of the box. The PPI is a must. Contact a local specialist if you are serious about the car and take it to them with the buyer. Research the PPI place on the forums too.
That should get you started.