Whats the score re. running costs such as fuel, service, tyres, road tax performance etc.
I get 22-23mpg round town and 37-38 on motorway runs. Tax is £200ish per year I think, tyres (non funflat) are about £70 per corner, can't tell you about servicing I'm afraid as mine has the TLC package. Performance is decent enough as standard but a £60 reduced pulley will give you around 200BHP so they're very easy to modify if you're so inclined.
What are the things to look out for on them?
Clutch (well, flywheel) seems a common fault but generally it's just the little things which all add up
Finally what are they like to drive and own?
Fun, fun, fun. Chuckable but not terribly refined. Better than the equivalent hatch but not a patch on your 330.
Will I be disappointed in my change?
Only you can answer that. Personally I'd much prefer the Mini as I love small chuckable cars but not everyone shares my preferences.
Just newly married and have a mortgage to pay.
I hope you're not planning kids as the rear seats aren't going to be much use after about a year. Rear seats won't be much fun for a regular sized human for any journeys that last more than about 30 mins.
Any other cars along this line which we should consider?
You've got the other obvious choices like Clio, Golf I suppose. They won't be as fun but might be cheaper to run.