Hi, Debbie,
Sorry to have been so long in coming back to you - my PC has been down for a few days.
I expect that you will have had the results of the ringworm test by now - I shall be very surprised if they turned out positive.
Explain to your vet that you are very anxious to buy this horse, and ask ( politely) if he would be prepared to carry out the rest of the 5 point examination, and if he finds nothing else, to issue a certificate with a caveat about the skin problem.
You could also ask for a second opinion from a veterinary dermatologist. If you are calling from the UK, all the veterinary schools and the EHT at Newmaket have dermatologists, and there are several in private practice spread over the country. Ask your vet for a referral.
It is unlikely that the vendor will give you the undertaking that you require - it is up to you to satisfy yourself by all possible means before you continue with the purchase.
Walter