Hi, I'm about to start my second year doing MMORSE.
1.They didn't really check calculators during the exams, so I wouldn't worry about it really. But just to be sure, if you only have a graphical one which can store alphabetical data, get a scientific one, which only costs like a fiver.
2. Yeh, it is probably best you take a module that interests you. I started out with those intentions but then just took "easy modules" which I think are seen as Stat Lab, Games and Decisions and Differential Equations A, especially if you have done them before. If you have done a lot of mechanics, do mechanics. Hopefully the MORSE society, of which I'm part of the exec, will do something after your introductory lecture where you can ask people from higher years about modules and the course, and meet other first years.
3. MORSE does have a reputation for being a bit hardcore, but to be honest I think the first year is not too hard really. It's just about finding how much work you have to do on a weekly basis. I managed to achieve a first (albeit I scraped it) without stretching myself too far. Look on the main applicants thread or the search function to find peoples opinions.