You don't metion what he wants for it so can't really say if it is a good price.
I have listed some bits below but i would suggest you have a goo look over the resto section at all the S1 resto's, it will give you a huge amount of info for where rot gets and will amaze you how what seems like a very tidy S1 on the surface is a rotten shit underneath. Get browsing.
Thing with an S1 is that rust is a big headache. Remeber that if the scuttle panel is rusty, then the bulk head may also be going, the screen will cost a bit to remove and it is a bit of a pain to sort.
Door aint so bad, my advice is to properly check the whole car for rust, and i mean properly. Get the carpets pulled back in the foot wells, check the inner and outer wings, check the slam panel and where it joins the wings, Battery tray, the rear valance on the joins, the boot floor. in the sunroof resses and the roof. Get on your hands and knees and have a good poke around underneath. Check the sills.
Also the trim can be expensive and difficult to replace so how good is it?? this also goes for electrics, door locking, windows, switches etc.
Engine wise, if it pulls wel with no knocking or smoking and if it is oil leak free and starts well from hot and cold then your doing ok, gears should go in easily, look for wineing or crunches as this will be rebuild time.
See if the brakes are upgraded as they weren't much cop, also see if it has upgraded suspension as the original would be tired by now.
Check how good the history is, make sure it all fits. just a stamped book and no bills aint much good.
does it have a cat 1 alarm, you'll need one.
Top motor though, you'll love it. Engines and boxes can be sorted, but rot can cost insane amounts, don't fall in love on the viewing and think oh that'll be ok, a battery tray done well could see off 200 quid and inner wings can be a huge huge job. floor pans go aswell so really really check for rot, oh and make sure it isn't a ringer.
So, how much does he want, some in the for sale section at the mo should give you an idea of what they cost
Matt