For battery reasons, I wouldn't buy a used iPhone 7 at this point in time. My iPhone 7 that I got on release day (2016) is at ~80% battery health and drains quite quickly even with very light use (I do most stuff on iPad). If I actually used the iPhone 7, I'd likely need to charge it 2-3 times a day.
How much will you be paying for service? Best Buy has $299 Total Wireless and Simple Mobile prepaid kits which is probably best pricing you can get on a brand new iPhone 7 with warranty.
https://www.bestbuy.com/site/iphone/...ce%20skuidsaas