As you've posted in the YORCC section, I'll assume you're only racing at York for a little while
Speedo and motor wise there are no restrictions in modified classes. Most sensible people with normal (rather than superstar) thumbs would recommend a speedo rated around 100amps and a motor around 10.5 for either 2 or 4wd. Plenty fast enough to be competitive in the top heats.
Cells - hardcase only, 2 cell only. They do not need to be on the BRCA approved list. Think Turnigy Nanotach - great cells and cheap as chips. 1 pack is fine, 2 is better. Don't forget a charge pouch and lipo compatible charger too!