I have a 186D, and a Shibaura 5 foot tiller (give or take). I haven't yet run the tiller on the 186D, but I have run it on an 18 PTO horsepower Mitsubishi. In my soil, even thoroughly and deeply disked, the 18 horsepower tractor will bog itself down and not be able to pull the tiller when it digs deeply, to the depth of the tines. This is in first gear, lowest range on the PTO and the transmission for the tractor. If I am careful about keeping the depth relatively shallow (say, 1/3 to 1/2 the tine depth) it pulls it just fine.
The YM186D is the same powerplant as the 1510, which is rated at 15 horsepower at the PTO. California has run his 56 inch Yanmar tiller behind his 186D Here:
http://www.tractorbynet.com/forums/yanmar/193622-1401d-vs-186d-3-point.html he told me this:
"RS1400 tiller (56 inch, I think): Too much tiller for this little tractor. Horsepower is sufficient if I don't let it cut too deep on the first pass. But this orchard is operated by my neighbor with larger equipment and I'm mostly doing the fence lines, around low trees, etc where he can't go. When I'm tilling along and hit a 6 inch step up from his disced ground to untouched ground, the tiller stalls just as the nose of the tractor climbs that step."
I don't know what your soil is like, or what sort of custom tilling work you will do, but I know that my 186D wouldn't do well with a tiller of that size. Mine also doesn't seem to run particularly strongly, but I don't think it would make that much of a difference. The 56 inch tillers originally came behind 20 PTO horsepower tractors, and were intended to run in muddy rice paddies, so taking a 25% cut in horsepower is a serious step down, particularly for heavier soils.
I'm not trying to discourage you, because it sounds like a good deal for a new tiller, especially with a spare set of tines. For the price I would try it. Around here I could probably get the money back out of it if it wouldn't work.
