as far as tax i wouldnt worry too much-if the seller has it taxed it wont be an issue as it wont ping any cameras etc
would make sure car is insured by the seller if you are driving it even for a test drive as your own driving other cars cover [if you have it] wont in most cases cover you...if its still insured you could write a wee chitty up saying for insurance purposes only seller will remain the legal keeper until you arrive home
the new tax system is designed to generate double revenue for every car sold on the last month-simples