I think the boot cleats are universal and mostly the SPD design. If you are going off road I'd probably suggest you look at a heavier duty boot with good foot/ankle protection. I mention this because I currently use a very thin Shimano shoe on my road bike, and I know for sure it is not adequate for the off-road stuff I want to get into. My existing shoe is not waterproof and that counts against it. I've seen some decent off-road shoes and they look sturdy and waterproof enough.

I've read reviews about clipless pedals and some mention to look at pedals that won't clog with mud or at least be easy to clean. It seems some clipless pedals are better than others though that's all I've read. I'm sure the folk who have done some dirty rides will have more practical advice.