Right...
Do not get another dealer car or new car, you have found out that they still have problems just like any older car.
You now have a cheap older car, which you prefer and realistically is cheap to fix compared to any newer cars.
Keep it, and use it as a daily.
If your so worried about it being nicked then fit some decent alarm system, decent steering lock, anything like that

Regarding rust, tough titties, but your first point of call after getting the bodywork finished is.. (what I think is a good idea anyway...)
~ Take the rear panel cards off, and modify a nice hole into the sills either side so you can put the sunroof drain pipes to go out via the sill behind the kit!, obviously get a nice little drain attachment or something fitted into the hole you have made, and obviously rust proof the hole first and seal it.
This will mean your sills wont rott as fast as normal from the shitty for design!!
Remove all signs of rust under the car and clean it up, Get access to use an air compressor, get an underseal gun and some cans of underseal, spray into every little area you can think of, in the sills, wheel arches, in the little holes etc etc even in the rear quarters on the inside and the bottom of the doors on the inside!
Make sure you wash your car properly, specially over winter
As long as you do all that and keep on top of cleaning it, I cant see rust being much of a problem IMO... if any does show up on the panels, just get it sorted after winter, it be reet.
Most of all, enjoy it and save your money!