Tax for an E30 is approx £123 for 6 months and £230 for 12 months, 1.6/1.8/2.0/2.5 etc all the same as it's before the 2001 tax changes.
A full wrap will vary depend from garage to garage depending on the type of vinyl and quality of the finish as well as if you want bits like the door inserts also done properly. A complete wrap as far as I know can be anything from £500-£1000. If you're looking for a cheap paint job try plasti-dip (or something along them lines) it's durable and also if you want you can peel it off to reveal the paint underneath when you're done with it.
Repairs wise the E30 can be quite a money pit, but they're all variable it's hard to say. Rust will be the biggest problem usually. There's a lot of spares you can get through here and everyone's friendly if you need help.
Check out these off the wiki for an idea on what you're looking for when buying
http://www.e30zone.net/e30zonewiki/inde ... ing_an_E30
http://www.e30zone.net/e30zonewiki/index.php/Rust
As for insurance, well once again it depends on this and that really. I got mine when I was 20 and insured for £1000 as standard, now it's insured with it's modifications and 3 points for £550 (nearly 4 years on).
Have a look around for a good deal, companies such as lancaster, sky insurance and adrian flux tend to pop up here a bit although adrian flux is a broker and will find you insurance through another company - but that worked out fine for me.
Personally out the two you're thinking about, get the 318is. Tend to not rot as much for some reason (or so I get the idea from here) and you'll enjoy the added power compared to a limp wristed 8v.