Hi Nicky,
It depends on what you are using the car for? If you are racing at BRCA events such as the regional series you will need an approved motor as per their homologation lists this applies to battery choice as well -
http://www.brca.org/content/homologa...-updates/14218
If just for bashing around or at your local club then just be wary of a cheap low amp out put as others have mentioned, you may not get the performance or longevity and it could be false economy.
There are usually a few top end speedos and motors being sold off here on Oople so it may better to buy a decent second hand one than a cheap new one.