Ok first let's clarify some of the basics, a race bike is going to be a fairly maintenance intensive toy regardless if it's a 2 or 4 stroke. Don't let anyone advise you one way or another for either of these bikes based on how intensive the maintenance is going to be. A 4 stroke trail bike such as the older Honda XR series or their current CRF F series will be much less maintenance than the competition models.
Based on the way you describe yourself for me the end decision will come down to what you plan to spend. If you are planing to stay in the used bike market I would look at used 125's (CR, YZ, KX, RM or SX). If you want to go new then I think the best choice for you would be a 250F (pick a color).
I think the best bike choice for your answers would be a 250F but I would never advise anyone to buy a used one.
The KTM 200EXC is a great bike and a great option if you feel your racing is going to be off-road versus MX. It is quite capable on a MX track but would not be my first choice for that venue just as a 250F wouldn't be my first choice for racing in the woods.