One thing you could consider: I began my all grain adventure doing BIAB with two relatively cheap bayou classic aluminum 5 gallon pots, which worked great. I could do two mashes if i wanted to do a really strong beer, or just use one pot to mash in and the other for sparging. When I felt like it was time to go beyond BIAB, I simply modified one of my 5 gallon pots to turn it into a nice mash tun. I bought the weldless brewmometer, a weldless spigot, a bazooka screen, and drilled a couple holes, and presto! I even slightly increased the size of the steam hole on the lid so that I can stick my long probed thermometer into the mash to get another temperature reading from a different spot. It's a pretty sweet mash tun. I then just bought a used 7.5 gallon pot to do my full boil in.
Just think about how you may want to upgrade your setup in the future and see if you can buy something that can be upgraded.