Hi, It looks like she has lots of bone and should turn into a beautiful driving horse. We have a three yr old filly that is currently in driving bootcamp. We decided to let a pro do the training for us as we only have experience with well trained horses and do not want to skip things in her training. We figure it is easier to pay than to fix. Mini's are very forgiving when it comes to training to drive but that just means that problems can go unnoticed.
You must know the expression about the green horse and the green rider, same thing for driving, My advice is to find a good instructor and get the right advice and guidance in the process.
People seem to worry a lot about the capabilities of the smaller horses, IMHO it all boils down to the proper training, harnessing, cart and then not to forget conditioning of the horse. Mess one of these up or underestimate the importance and the whole thing can become a terrible experience for your horse and ultimately for you.
Take your time get an instructor be prepared to spend some money for the best gear you can find, quality always pays of in the long run!! and I am sure your little mare will give you years of driving pleasure.