I would add another shirt and pair of pants so that you have a backup and don't have to necessarily always do laundry right after an evening of games.
Do you have a jacket?
Also, check around with some local folks to see if you need to have v-neck undershirts or not. In some areas, it is frowned upon to have your undershirt showing underneath your stripes.
If you do need v-neck shirts, I highly recommend these.
Diamond V-Neck HOTSPORT® Loose Fit Shirt - Black: Ump Attire
I also recommend the Ultimate Referee shirts sold by Ump-Attire as well.
Welcome to the hardwood. I worked baseball for around 10 years, plus two years of football before starting basketball last year and I had a great time doing it.