I expect there are regional differences, but here a 4 to 5 foot cutter is more common than a 6 foot or bigger unit. But, if one buys something that is reasonably priced, it should be possible to sell it again for about the same amount, too.
The used equipment market seems pretty consistent, and the rapidness of craigslist searching means that things priced below market value sell virtually instantly, and things priced above market value linger interminably. For instance, 3 point discs around here are about $350 to $500 dollars. Much higher than $550 or $600, and they never sell. A unit priced at $250 to $300 is gone within hours. Rougher looking units sell slower,or for less, obviously, but a coat of paint is a cheap and easy way to boost perceived value.
That means with a bit of time spent looking (over, say, two weeks, or whatever seems appropriate for the turnover you see) you ought to be able to get a feel for what the value is of a given implement is for your area. Buy with the intent of being able to get most of your money back, and you should be happy. Shop with the goal of being able to profit after using it extensively, and you'll be perpetually disappointed.
If you bought one at $400, used it 4 times, and resold it for $350, it only cost $12.50 each time the implement was used. That's still a good investment in my book, since there is no way to rent it for that cheap, and isn't much off a six pack of good beer if one borrows from
a neighbor and repays debts in such a manner.
Since it sounds like you're experienced with tractors you likely know all that, but it's worth putting out there anyway. I'm glad I don't have to mess with the brush you describe. That sounds as if it would get old very, very quickly...