I would never recommend that a client rely on an email as the basis for a contract, let alone a multi-million dollar deal.
Entering such an agreement without the assistance of a competent contract attorney is an invitation for trouble down the road. Courts will often look at emails and other 'extrinsic' evidence to help determine the intent of the contracting parties, but such informal correspondence generally leaves the court with too many questions unanswered for the email to stand alone as a binding contract.
I agree with the previous answer. It is best to have a contract setting forth the entire agreement. I would also consider having the sales money placed into an escrow account to ensure your payment - especially where the seller may have some period of time to review the books or otherwise ensure the purchase is appropriate.
I also suggest that you seek the advise of an experienced contract attorney to avoid future litigation before it's too late.