@ the OP
i find it odd that you would rather not reveal too much about yourself and your finances and consequently you'd rather not answer questions like these:-
The_Dark_Side said:
how much of a percentage of your income do you want this to provide?
how much capital do you want to invest?
how much effort do you intend to dedicate to this side of your business?
then you go on to say you are about to rent an old dealership AND that you can easily make an extra £500-£750 per week simply by increasing your hours at work, clearly indicating that you're wealthy.
seems a tad contradictory to me but hey ho.
to be honest to pull more than a grand a week profit is going to take a fairly high level of commitment both financially and in terms of occupying your time.
if you can increase your income by the amount above by merely working more hours then i'd be going down that route.
as Fox said, to sell expensive looking cars you need expensive looking premises, you need staff, backup etc and alll of these take money to setup and run thus cutting your profit margin per sale.