I prefer Bio to leather, weighs less, but neither is light when lifting above your head. Bio is also easier to clean a little 409 and hose it off and hand to dry. Leather is wash and condition and try and keep the mice for eating it up because of the oil.
Has your guy been driven before or just ridden? If he has then it should come back to him pretty quick if he has only been ridden, it a total new concept for him. Make sure he's comfy just walking around in the harness in a round pen to make sure he's ok with the feel and noise of the harness. Also make sure he understands pulling before going in the woods as in pulling a log/post around in a enclosed area.
Not sure if I can give a couple of names and phone # on here or not, so send me a message and I can get you a couple of folks that can help you with a harness.
Your choices are leather, Bio, & Nylon.
Leather- Con. heavy, extra time to clean (unless you use it on a daily basis, then it conditions itself with the horse sweat)
Bio- little lighter, cleans easy with spray cleaner and hose, drip dry.
Nylon- lightest in weight - not the prettiest to me, but I can't say much about them havent ever had a nylon harness, but the cheapest to purchase.
Your going to spend at least $500 on a leather or Bio, nylon you can probably get for around $400. That doesn't include a collar, which depending on what size you need which will run $100-200. You need to know the collar size, the hames on the harness need to be 2 sizes larger than the collar.
If your going to be logging you want a work collar not a driving collar, two different things. A driving collar will be cheaper, but is not for work just pulling a cart on smoother flat areas.
Hope I haven't confussed you to much.