Hey Native, it's me again. You can get there all of 15 min ahead of start time or right at start time, doesn't really matter. You only see fundamentals and position drills for a while at the start (usually). After 20-30 min, they break the drills for team stretching exercises, then break into team drills, so you don't miss much right at the start.
The Pro shop is right at the entrance to the bleachers, and yes they have a lot of stuff, but be sure to bring your wallet (and maybe someone else's too). Nothing is cheap, nor is it cheaper than anywhere else, or any other time during the season. If they run sales, I have never heard of it. (Maybe they have sales on Culpepper jerseys now, I don't know).
Anyway, have a good time. I am very fair skinned and sun-phobic so this may help or not...If you plan on staying they whole day, take some sunscreen and a white towel, and make sure you have sunglasses and a big hat. If you are not used to a lot of sun, you will definitely fry for awhile if you are not careful. I usually put a white towel over my lap while watching practice, and I can tolerate a few hours of that without being uncomfortable. All depends on you, and the sun.
EDIT: Binoculars are not really necessary unless you have poor vision. You are literally right next to the field. I was only aware of three evening practices, unless they changed the schedule. Yep, looks like Tuesday night might be the night for me to go too. Also, since you're going at night, consider that a lot of the fans can only go after work, so it will probably be a full crowd. This is one you might consider getting there twenty minutes early, just to beat the parking congestion above all else.