Best thing for them to turn up in is overalls and boots then if you like the look of them you can tell them to get on with it
I'm not sure that would go down all that well with the majority of potential employers. I agree with WnW that a suit is quite possibly OTT, and could be an embarassment, however turning up in something reasonably smart is a mark of respect to the employer. Bringing wellie boots and a boiler suite to change into if needed might well give a few extra brownie points though: you can always mention you've got them, even if the interview isn't the sort where getting stuck-in isn't appropriate, and you don't need them.
Qualifications can be a quick way of telling the employer what he might expect from the candidate, but it doesn't do to either insist on them or equally to expect the candidate to be able to do things mentioned in their qualifications in the way that you as an employer wants. There are few agricultural tasks that cannot be learned in a few hours with the right attitude and a bit of effort. Equally, the over-confident know-all with a string of qualifications is a disaster in virtually any type of employment. "I'll have a go if you can show me what you want doing" , is a very positive statement, and difficult to fault.