Thank you, both of you, for those comments so far.
It's a 1999 by the way, so if the seal lasts only 5 years, then I guess that's why! They said that they'll do what it takes to get the damp issue sorted out and make it leak-free (not that I can really check they've sorted it unless I use a pressure washer on it before buying lol).
Also I told him that I would want the aerial replacing, which he agreed to do. So that covers those problems.
I just don't know what else to check. All the stuff seems to work ok, and I will be assessing it carefully on the test drive tomorrow. I also checked the transmission fluid and engine oil. But I wouldn't know what else to check in terms of under-the-bonnet stuff. I believe that a whole new exhaust can cost £2,000+ so I will attempt to see if I can assess that.
They are asking £9995 for it. It's done 71,000 miles, full history. Seems to be in excellent condition as far as I can tell. I am hoping that I can get it converted to LPG for around £2300 and then keep it for 5 years+.
Just one question though, when you say that I should try to find out if any of the pulleys or tensioners need replacing... are they not replaced at standard intervals along with the belt? Are they only replaced if they look like they need replacing? If I ask them if they need replacing, they are bound to say "no" as they know I would ask them to do it before I buy the car.