You will probably have to PAT test EVERY SINGLE LEAD (including mic and guitar leads) you plan to use, and every item of equipment to be connected. The usual cost is £20 per item (!!!) That alone makes it cheaper to get a proper hire company to do the PA.

But 600 watts is not a lot for a marquee in the open air even for a largely acoustic thing. The most important thing is to remember not to turn up too much if you don't need to. I have seen an acoustic player with a mandolin (a piercing instrument at best) unable to compete with the noise from drunken Henry and Harriet in a pub despite a 1k PA rig. But that ought not to happen at a Folk fest.

What date is this event - it sounds like a nice amateur vibe to it (and I am an old fart)?

What is the fringe like, the sinagrounds and stuff?