Gray market typically is foreign manufacture with questionable parts support, you see it with importation of used heavy equipment, and have to check serial numbers to determine what it is. I believe it could apply to some or all of the smaller/ ford models, not 2000-8000 and up series, just the compact and sub compact models. I don't see it applying to a 5000 or any thousand series of the older designs.
We had a previously owned 5000 in stock one summer and put it to use baling, good match all the way around, but would be cumbersome in some places I worked with my 850, or the '64 4000,(4cyl)way back when. I've run a 5600 in the past, with a loader, at a friends place working part time. It was a little big, but very capable, he still has it today. Years ago it was listed for sale, a one owner since new, I thought about it as I remembered what it was like to run it, that and its a well cared for, always been in a shed, nice original tractor that has thousands of hours left in it and then some.
I think the prices of the models you are looking for have come down, seems 10 years ago, these still brought more money than now. I see them around 5K + or - locally in various condition.
I used the same rationale to upgrade as my 850 is well worn, engine runs good, but it needs a lot of repairs, it has served me well for 13 years and proved to be one tough tractor.
I could not justify something new of a 4000, 4610, 4630 class tractor either, its just way out of my budget. The solution was finding a low hour 90's model. I settled on a ford/new holland 4630 fwd that came up locally from the 2nd owner. It needed a lot of minor things and small repairs, but its all the tractor I'll need for the work I need to do. Instead of plunking down 5K for something older, needing more repair, I took some of that and used it for a down payment to finance this upgrade. I lucked out as it went on consignment at the dealer that sold it originally, this made financing easy. I'll own it in less than 4 years now and its a modern tractor that covers my needs. It has good parts support,(now at least LOL!) and it does not have 10,000 hours on it. It was under 15k, and is less than $300/month.
So far, I think I did just fine with this purchase and I really appreciated the advice or comments from others here that helped me with making a decision, both technical and practical. Who knows what could happen with it long term, like any piece of equipment, new or used, there is risk. You do the best you can, with both the purchase and the maintenance you give it. Once I tried it out, what a difference, really something to appreciate and after 13 years of the same annual work, sure is nice to have power steering !!! LOL !! FWD, wow..... that's all I can say. No loader but I'll get one for it eventually. I'll just use my old one when I need to.
I have more acreage to cover, just under 100, but its not all fields, as some are over grown and wooded now. There is no doubt that what I have, will make things easier, its a perfect size for me, fits in a standard residential garage too. Fold down ROPS is perfect for low clearance. I don't need a cab yet, so this one really matched up to get into the garage. and it does have what turns out to be a factory correct under-slung exhaust, no limbs to knock the stack off or whip you across the face. All around good size tractor for a smaller farm or operation, no doubt about it.
