Welsh are pretty much free of genetic diseases, however there is some incidence of glucoma. If my memory is correct an eye testing was held at a breed club show a while back and all dogs passed with flying colours.
This is a very small breed and by and large the only place you'll get sound advice is from within the breed. You say that you don't have a dog yet but are perhaps interested in a future within the breed, if so choose your breeder well as they will be the best person to advise you in these matters. Find one who is willing and has the time to help you, go to some of the Champ Shows this summer and talk to as many folk as possible. I'd also advise getting hold of a copy of Bardi McLennans' Welsh Terrier leads the Way.
Sorry it's take so long for a reply, there just aren't many Welshaholics out there!