Hi, I recently purchsed a 2000 model C2 coupe after an 18 months search. As Richard pointed out, yes - if it is a private sales the warrantee (assuming it is an official OPC warrantee) is transferrable - there's a form in the back of the warrantee booklet for the previous owner to complete. So, although a 111 point check is not technically necessary for the warrantee, I'd get one done anyway (only about 130 GBP) and attend the inspection to see what the mechanics think of the car. As well as warranteeable items, they'll tell you about the condition of other parts too (e.g. tyres, exhaust, good look under car where you can check for accident damage) as well as give it a test drive and overall impression of whether the car is 'nice and tight'. More importantly they'll put attach their diagnostic equipment to the car to check for any fault codes etc. There's always a possbility if you buy a car without an inspection that it has been (for example) overreved in the past which will show up on the diagnostics - if this has happened and your engine goes pop the warrantee will be void. The OPC will also be able to verify the service history. And also, their diagnostics will give a read-out of total running hours which can be used as a cross-reference to see the the mileage is about right.
As for number of owners, that's a difficult one. The general wisdom is that if they have all been careful or enthusiast owners then more owners is not a problem. Some say that if the car has been resold that many times (e.g. once per year) it will have had any problems sorted out each time it is sold, which is a good thing. The danger is that 1 or 2 of them may have bought above their budget and not been able to afford its upkeep. However, many buyers (myself included) prefer a low number of owners - purely on the subjective feeling that less owners means each owner has had the car longer and (probably) cared for it more. This WILL affect the ability to resell it and the price you will get. I looked at a 98 model that had 6 owners, 55,000 miles and was on sale for 23,000. I didn't buy it and despite it being passed on to a porsche specialist, remained for sale for about 9 months.
Anyway, from what you say, if it has a full OPC service history, is a C16 car in the colours you want and the 111 point check comes out OK, then I'd make an offer between 24 and 25K for it with a new MOT. Quite a few people will be put off by the number of owners, so I'd be surprised if there's a rush to buy it - do you know how long its been on the market and/or why the current owner is selling ? Other factors affecting resale of older cars will be manual vs. tiptronic (tiptronic fetch less than manual) and colour (black, silver, dark blue are more popular (hence easier to sell) than red, green or yellow). Similarly Boxter red, mint green or savannah interiors are less popular than the greys, metropole blues and black). And I guess, for cabriolets, the time of year [mind you haven't we just had our summer?].
I'd be prepared to go nearer to the asking price if the tyres are quite new (and correct 'N' rating) and if its had any significant work done (e.g. if its already had a new clutch, that would be perfect).
BTW: A little bonus for pre March 2001 cars is that the road fund licence (aka Gordon's cash cow) only costs 180 GBP per annum.
Let us know how you get on.