Here are some answers hope they help.
1. a 1.4 16v Rover would be in the Stockhatch class. There have been a couple of those in the past, the most successful being Simon Horton, he got the car well sorted and had some good results.
I you wanted you could do some, engine, body, and suspension mods and run it in the Supermodified upto 1600 class. Brian Thornton runs one of those, but I'm not sure what engine he has in it.
2. The nearest tracks to you would be Lydden Hill in Kent, or Mallory Park in Leicestershire.
3. The next meeting is 24th March at Lydden
4. In Stockhatch the diff has to remain standard. In Supermod you are allowed to have a LSD. A locked diff would not be advised as we have to turn both left and right, so stagger is not used
5. At the moment entry to a Clubmans Championship round is £200-£215, although winter series meetings may be cheaper.
6. Unless you are an expert fabricator, and have experience of building a MSA spec rollcage, I would always advise buying one, or visiting a specialist manufacturer.
It's probably safer, and definitely less hassle at scrutineering