During HackJam this week, we had the visit of Ben from Toronto Mesh. He introduced us the simple and easy way to construct a mesh network based on a network of a few Raspberry Pi2 or Raspberry Pi3.
They use the model 2 or 3 from Raspberry Pi because the encryption of the data transfer is CPU intensive, not memory intensive.
The lower the CPU is on the board that you use the lower the data transfer speed will be.
So a Raspberry Pi B+ can be used, but the transfer rate will be just a fraction of what you obtain from a Raspberry Pi3.