1) expensive roof - you mean a new soft-top? Very expensive from BMW, otherwise prob best to budget 500 for worst-case scenario (new front seal, new actual fabric, new springy-things that make sure the hood folds properly, professional fitting, sundries...)
Hard-top is circa 800
2) are they really that bad in winter? No of course not - it's a BMW! You can't scrape the rear window, and the special blower takes ages to clear it, but who looks out the back anyway?!
(disclaimer - this was an irresponsible and crass comment made by an idiot)
You can get a special cover for the rear window that clips-on overnight in the frost, and you just remove it in the mornings - I just never remember to use mine, and I have a hardtop anyway.
Too cold? The heater does a very good job, but I do tend to find that above 60mph, all of the warm air being pumped-out by the heater escapes the vehicle (top-down of course)
3) I only get 25mpg but that's a short journey of 12 miles on country lanes with lots of accelerating and stopping.
Your only downside is that if the hood is frozen or wet, you can't really put the top down, but you certainly can't take the top-down on a tin-top, so it's all good news
