Isn't the minimum a good arrangement of tells on your sail and a stopwatch? How serious is your racing?
I don't do very serious racing, but I do try to get better at it. I usually get to the line
half an hour before start so that I can figure out which is the favored pin and timings (based on current wind conditions) for getting there. That is why the stop watch is helpful.
Sailing the same boat time after time will help you get an idea of what AWA's it is fastest at. This is going to depend on sails and sail condition. A handheld GPS
measuring VMG may help you understand that, but on a small race course I wonder if you'll find yourself looking at it during the race. We use the GPS
for speed over ground measurements too. You don't need anything fancy here and our GPS
is normally just reporting two things: speed over ground and current time. We don't have coordinates for marks programmed into the GPS
because they move a little bit on every race.
A windex is helpful to double check what the tells are telling you, but it is nowhere near as useful or important.
Anything additional seems like gravy.