Check out this link may be helpful for you MINI Cooper Forum - FAQ & DIY: MINI DIY & FAQ - ALL GENERATIONS
I am not shore about the auto boxes, I would think you would get better MPG out of a manual box I here the auto's are quite heavy on fuel but I don't have first hand experience with them.
but my advice for the car in general is to get the latest one possible with in your budget. They changed some parts on some models around 04 so if you can get a car with a build date after July 04 if it's a cooper or one with manual gearbox.
The minis come with so many different options, so I would work out what you want and try and find a car with as many options as possible. There are to many to list.
Things I wanted on my MCS
A/C climate control
Limited slip diff ( a must for a cooper s I think )
Viability pack ( heated windscreen, mirrors, head light washers )
Leather seats (full or half)
But on a second hand car it's hard to find it all in one place I had to look a few to get my minimum but I am very happy with what I have.
Thing To look out for if you can
full service history plus bills of parts and work carried out
Front wish bone bushes,
power steering pump, do fail but you should be able to get them replaced by mini under a good will clam.
cam chain, lifter rattles or supercharger rattles when cold start
Right hand top engine mount any oil stains on the chassis leg means it needs or has been changed.they don't last long, also check the lower engine stabiliser to
Clutch rattles and noise at idle and pulling away
Get the car started and test everything in the car works: windows drop when you open the door and lift when you close them remote locking etc.
Plus all the normal car stuff brakes, lights exhaust etc.
Drive it for about 30 min and play with everything electrical
Reading back it sounds bad but I have covered quite a few miles in my car never had a problem, and still love driving it.
Good luck and I hope it bring as much enjoyment to you as it has me
Hope this helps