Definitely don't need a last gen. Someone else did the math upthread and came to one request every 20 seconds, which if you factor in burstiness and that if you have a particularly bad burst that slows down the system a little, the next request will take even longer etc. (ask me how I learned that lesson), it's probably good to budget for it handling multiple requests per second. For this application, my understanding is you've got a handful of people in your group that you're splitting a couple of expenses with, so data processing is small beans and that'll definitely run on a first gen Pi if you optimise it properly, or perhaps a 2nd-3rd gen if you don't want to spend the time