Yanmar did sell new in the US in the past, but not now. The YM followed by 3 digits indicate a US model, the remainder are grey market. Support for the US yanmars is good. Support for the greys is through networks such as this. JD dealers aren't much help although a few will try. Yanmar is trying to cut off parts for greys through the US dealer network in this country--perhaps because of lawsuits as the grey market ones don't have the required US safety equipment (Roll over protection structure--ROPS, seat belts, or PTO shields). Parts are certainly available and some models have very close US models and these models are easier to get parts for (although you may have to claim you have the US model and provide a serial number). Grey market tractors tend to be cheaper and if a widely made model parts should be easy to get--although you might need to do the work yourself or make friends with a tractor mechanic. Certainly the parts are available, but sometimes it takes time. A few models were hardly made and parts are a real problem--ask people on this board before you buy. The F series tractor are newer than the YM tractors. Many models use very similar/common parts. Also, JD parts sometimes cross-over and I think there is a JD website that crosses over yanmar parts number--of course you have to have the yanmar part number. Mostly you get a parts manual (in Japanese with Arabic numbers) and look at the pictures to get the part number.
If you want a new tractor, people are saying good things about Bransons, or you can sell one of your children for a JD.
Greys have the good points--mostly the price. You need to be careful about "reconditioned" or "factory reconditioned" as you have no idea what you are getting--maybe just a paint job and a rolled back hour meter. Many people worry about the vietnamese reconditioned tractors for this reason.
If you deal with a reputable dealer (and those on this board know who is who) you can get a good tractor for a reasonable price. Ask about a dealer on this board, put your e-mail up and you'll get private responses for negative feedback--the good responses will be posted for all to see.
Bigger machines tend to have more hours. This is not necessarily bad. lack of use is almost harder on an engine than lots of use.