Here are some very good kits out of the box (requiring intermediate level skills) that I could recommend:
Tamiya 25174 Firefly VC Sherman (ex Tasca)
Tamiya 25175 M4A3E8 "easy eight" Sherman (ex Tasca)
DML 6637 M7 Priest, mid production
DML 6627 M7 Priest, early
DML 6760 Sexton II SPG
DML 6793 Sexton II SPG w/CDP tracks
AFV Club Churchill tanks (many to choose from)
AFV Club M3A3 light tank
AFV Club M5A1 light tank
DML US Halftrack kits (not a tank, though)
Other Asuka/Tasca M4 Sherman variants (plenty to chose from)
Easier kits:
Tamiya M8 or M20 armored car
Tamiya Cromwell or Centaur
Tamiya M26 Pershing (the Hobby Boss Super Pershing represents one tank -- and is only OK, IMHO)
Academy M3 Lee or M3 Grant
Academy M12 Gun Motor Carriage
As for Ikar's suggestion of an M30 Cargo carrier -- that would require a scratch conversion of the M12 GMC and plenty of reference material. Not unheard of but a poor suggestion for a first venture into US or UK tanks, IMHO.
There are plenty of older and cheaper kits available (Italeri e.g.) but you get what you pay for. For a first taste, I'd recommend getting a very good base kit. You don't want your first bite to be all fat and gristle! hahaha