I have 56 PTO hp on my tractor and run a JD 375. I can feel it loading up near the end of the bale; not a lot, but you can tell the tractor is starting to tug against the load. Course the hydraulic ram pressure on the bale (to make it tight) has increased to around 2500# by then and that may be the source since it is only 900# before the bale forms and increases as does the bale and obviously the weight in motion has increased.
A 375 is a 565 (5x6) that only rolls out to a 4' diameter rather than a 6' so that you don't confuse it with the narrower 465 balers that roll a narrower bale, possibly up to a 4x6......not comparing the mechanics of the balers, just talking about relative bale sizes.
The 375 is all mechanical in that your pertinent information is on the front of the baler, mechanically driven. If you have outside mirrors on a cab, you can watch the level indicators and bale size indicator without having to turn around. Bale tieing requires a second hyd port on the tractor (gate the first) and there is an indicator on the front of the baler also to show you where the string is in crossing the bale.
In buying a bailer when I went shopping I essentially went looking for a 4xx of some sort; x5, 5.5.or 6 "gestimating" what this tractor could handle. None were available but with what I know now I would have stopped looking above a 4x5 if I were selecting a narrow baler for the PTO hp I have available.
You can do the math on volume to see what the corresponding bale weights would be considering everything else constant. 5x4 and 4x5 are pretty close with the 4x5 being slightly heavier.
I have mixed emotions about long (5') with a reduced diameter vs narrow (4') with a wider diameter. I like being able to see around the bale when on the FEL. A 4x5 would be ok too. A xx6 just gets in the way; did for me when I ran a 5x6 with a 100 PTO JD. A 4' wide would cure easier as it is narrower allowing the heat exit path to be shorter. When stacking you could put more bales in a given space. A 4 might tie faster.
Price and equipment condition are always up for grabs. I paid $5k for mine at an auction house which made retail sales before the auction, which is what mine was. It's a 1991 model with reasonable wear and tear. All the belts are 50% up to 95%. I think the 50's were OEM. I had no mechanical repairs to do. The chains were well rusted, but I got some Justin Bros chain lube (really a super chain lube) and have had no problems now in my 3rd season with it. Of my choosing I did replace the wires that tie the belt ends together and replaced the little rollers on the tensioner arms that tell you how level one side is vs the other. I shot Liquid Wrench Chain Lube on the ends of all the rollers and repeat every time I use it. Whether any of it gets past the seals to the bearings I don't know but all is quiet when running and nothing gets hot.
A little long winded, but you asked and here's how it went for me.
Mark