Geoffo:
Thanks for all the advice so far.I think one of the biggest problems I face is getting over the fear hurdle of venturing into something new.
I’ve had a straight forward run in up to now.Same job pretty much all the time.Bald. I have direct work with the truck I have. The other vehicles will be for a completely new ( to me) project.
It may be wise to get another customer, not all your eggs in one basket and so on…
Something new should be nothing to be afraid of, it might even present other opportunities in the future, but it may also present a few hurdles you never tought of, as it is completely new.
What I see, as important, is :
1: you will be financing truck, diesel and wages for a while, untill your new customer pays, can you afford that without stretching yourself financialy?
2: do you know a few drivers who you think would be an asset to your company and are they willing to work for you ?
3: how well do you know your new customer, and can he (they) be trusted to provide work for a number of years (the time to pay for your new (secondhand) lorries )
If the answer is yes, go for it 
If it is a no for one of these questions, beware and be very carefull 