I've done somthing similar onsite with photos.
I used CorelDRAW to personalize and print the photos.
I'd suggest having a limited number of shirt colors.
Display actual printed design samples showing editable elements that can be chosen.
Don't give too many options. Takes too much time and harder for folk to make a decision (as in a group of 6 girls wanting 6 different colors, names, fonts, print locations and costs and trying to put together money to pay for it all)
A back-up printer if possible. Extra ink cartridges.
Someone to help you.
Make sure of eletrical hook ups. And if outdoors have a generator for back-up.
A form for customer to fill out (themselves) noting spelling, color(s), fonts, details.
Get a deposit.