welcome to the zone mate,
i bought my first BM when i was 18, 2 door 318i in white, and when i paid the guy he said 'now you will never drive another car' and he was right, i still drive BM's now 2 E30's in my garage too. The 318i is underpowered when compared to a 325, but compared to a Nova its powerful. As you already have a car you can take your time, 800 quid will probably get you a nice 2 door 318 , and if you can go for the plastic bumpered face lift cars. Take you time and look for a nice spec with electric windows, power steering, sunroof etc. 318 engines are good for 150k+ miles if looked after. By the way look at my picture below, the 325 near to you is the pre-face lift, and the cab is the facelift model, note the rear lights are different.
Things to check - rust in the boot, open the boot and have a good look at the rear panels, remove the carpet and rear lightbulb clusters (you need no tools to do this)and have a good look around there, look in the spare wheel well, look for rust around the boot rim and near the boot hinges. look at my 325 right hand light in the bottom left side and you will see the dreaded tin worm has been at work(something i need to get round to fixing)Then get underneath it, check the sills etc. then of course the engine bay, i am always weary of a very clean engine bay as it means they probably have only cleaned it for the sell, so water will get into the electrical connections etc, better to see a clean engine bay but one that has been used, i.e. natural!
go for alloys if you can, cheapest are the bottle top 14 inch, which i find look nice if they are cleaned. If its style your after bargin down to 750 then spend the other 50 quid or so on some lowering springs and you will have a nice motor, something to build on and play with.
as for 325 and 320 they are thirsty on fuel and the 320 isn't much faster then the 318 but thirstier. but that also means that there are 320's about owned from new never thrashed and in good nick. I doubt you will pick up a decent 325 for 800 quid.
as for running costs parts are cheap and available compared to most other cars, with places like Euro car parts offering cheap non-original parts, which are usually the same thing in a different box.
let us know what you go for
mrlee
