Looks like this one has already sold, did you buy it?
You're no doubt aware the 33 and 164 are quite different motors, the 164 being the big flagship Alfa until the 166 came along where the 33 sits more in the middle of the range (or did).
I'd agree with the others, rust on the 33 is going to be the main problem (unless you're handy with a welding torch, which I'm not), you'll need to certainly check the jacking points and the underbody well, if the wheel arch liners are still there looking behind them would be advisable as well as in the engine bay, the suspension is another area particularly the front struts. In 1991 they were still using a 4 bolt system for which parts are still available but the top bearing ring wasn't a brilliant design and does collapse (fortunately also easy to replace and not expensive).
Otherwise check things as you would any other car to ensure they are in working order.
There aren't many 33's still on the road today, so if you take it on it will be your duty to keep it roadworthy!
Fortunately most of the parts are still available either used or new and there is little you won't be able to do yourself if you're okay with a spanner. To assist the 33 community is pretty active and very helpful so you'll find advice on most things.
I have 2 33's, one is a racer which I'm working to get back on the track the other I bought as spares and have yet to see (not yet collected), so can't offer any advice on what you need to look out for interior wise (the racer doesn't have one of those any longer) or electrics etc.
As to whether £800 is a fair price for the 33, that is very hard to say, very very very few roadworthy ones come up for sale, most of what you will see available are spares or repairs ones and depending upon what's wrong with them seem to go for between £200 - £500 so £800 for a taxed and MoT'd runner appears a good deal.
p.s. here is the orginal thread for the car http://www.alfaowner.com/Forum/alfa-...omeo-33-a.html