Hi admiralDog!
Welcome to the forum, hope you'll find the info you need here and if you don't find it feel free to ask!
Go for the 5mm!! no doubt. You need MOD1 pinions.
here is the sheet for selecting the right gears. There are 47, 49 and 52 spurs available from HPI.
If you are planning on running 14 cells, i would suggest using a 12 tooth pinion (Mike has got these normally in stock) it will go about 35 mph. you are going to use the stock two speed transmission with the mechanical brake?