Hi Vitamini,
I'm not an expert on slot Minni's although I had a real 1600S mini in the 90's, I had lots of fun with that car traveled all over Europe with that car, but a real bugger to work on.
Since it will be your first slot scratch build, why don't you keep a look out on E-bay or an other secondhand market for an old mini, you can do real bargains. With some milliput you can reshape it, how ever you want. For the powerhouse under the bonnet there are better experts than me. Good luck with your project, keep us posted when you start with the project.
Cheers,
Danny