Hello mate, welcome to the fold.
Very sexy Mini, and the Civic isn't half bad either for a rice burner
I'm a Surrey boy too, so good to have more local boys with 300's.
Most of your questions are hard to give a straight forward reply as it all depends on what you are after. Rule of thumb (i think) is the more letters after the number, the better the spec of the car. The Top trim level is the 360Glt which has the 2ltr, but as you say your age may hinder insurance. I think the GLS is the same but carb rather than injection. I say i think coz i'm not sure but i know one of the anoraks will be along to correct me shortly

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Mechanicaly they are pretty tough old tanks with the 1.4 having a timing chain rather than belt so less prone to breakage. Carb bases can cause running issues on 1.4 but mostly affects the 1.7. Rust wise they are getting on a bit, so just check the usual areas you'd normally look, wheel arches, cills, front wings/valance, floorpan, chassis rails these tend to go near the back where it splits in two. I think they are fairly rust resistant compared to a lot of cars of a similar age, but that said some can and do rust, badly. Most body panels are obsolete or hard to come by.
Hope thats some help.