It depends what diseases as some are not carried through eggs, but MG is spread by hen to egg. Unfortunately, quite a few poultry breeders here in the UK have a serious lack of biosecurity.
Be upfront with the breeder, look at their stock as best you can for runny noses, watery eyes, pale combs on layers that are not moulting, any gaping of the mouth and standing quietly away from the flock. Disease in eggs carries lower risk than the bird itself, but it’s still there. Birds who are carriers may not be displaying any symptoms at all, so you’ll have to make as good a judgement as you can; if they are actually letting you touch the birds and walk within the pen, without giving you shoe covers/overalls, that’s a big no no.
But overall unless the owner has their birds regularly tested and shows you the papers, there is no guarantee they are not carriers. I’d advise the above and disinfect the eggs before incubation to minimise the risk.
I was in the same place as you and had to cull all my flock, I now only buy eggs and raise any chickens I want that way; I also keep them in a netted enclosure to minimise contact with wildlife.
