Welcome to TMF Stephen.
You have certainly come to the right place if you need information and advice on the classic Mini. 
There are plenty of Mini's to choose from but as you are already aware, the suffer from rust quite badly.
when viewing potential cars, try to look for an 'honest' car that if has had repairs done (most have) that they have been done to a good standard.
If there is one thing that tends to let Mini's down, it is bodge repairs.
The common rust areas are in the seams under the headlights, the sills, inner sills, front floors, heelboard (where the front of the rear subframe bolts to) and the boot floor.
Basically look everywhere you can for rust and bare in mind that rust in seams tends to start from the inside so any bubbling of the paint from the outside is usually the last step in the rotting process.
A common cause for major rot in the floors and sills as well as heelboard ends seems to be the fitting of cover sills which is regularly discussed at length around here and can cause some shocking damage to the structure of the car.
the cover sill is a repair panel designed to fit over the top of the original narrow type sills and fitting of them results in trappeed moisture and accelerated rusting of the key structural parts of the bodyshell.
A good thread all about cover sills can be found here http://www.theminiforum.co.uk/forums/topic/290598-oversills-what-they-are-and-why-you-want-to-avoid-them/ and is well worth a look as these can hide some of the worst nightmares and most expensive repairs on the car.
It may also be an idea to look through the technical section of the forum as this will give you the low down on common mini problems and what is involved in repairing them.
Another thing to think about when looking at Mini's is to get the most solid rust free car you can afford as generally speaking, mechanical repairs and issues are easier and cheaper to repair.
If you are interested in the body side of things, have a look at the projects section. there are lots of great build threads within and will give you a good insight into the common repair areas and what is involved in repairing them.
Good luck and feel free to post any questions you may have and we will all be glad to help.
Cheers
Ben
im going to test drive a 01 very last of the classics tomorrow
