Build or Buy? Build can be cheaper but it will take some precious time. I bought a used high quality cart and never regretted the move.
 
Custom blend? I think that’s a bit over the top. There are a lot of great brands out there that match every flavor and texture preference. Try some out and carry one or two on the cart.
 
Soda’s and syrups? Once again I would focus on the dogs. Seems that you’ll spend more time dealing with the sodas than you will tending the dogs. Think of the waste that you might have in having to carry all those syrups.
 
Software? I run a small restaurant, in addition to my cart, with an Excel Spreadsheet and Quickbooks. You’ll spend more for your software than you will for your cart.
 
My advice…start out simple, focus on quality dogs and great service.
 
Good luck!