OP - You need to think about the physical size machine you want beyond just under 40hp... I've got a JD300, and a MF 202 Workbull. Both of which are 45ish hp, but are full size industrial machines. Loaders are in the 2000 lb lift range. The Deer is a 65, and the MF is a little older. Both great machines, but I wouldn't really be looking at quite that size if I was on an acre or five... On the other hand I have a couple Case 646's, one from '75, and the other a later 80's. 16hp, and the size of a heavy garden tractor. Also industrials, with no-nonsense 700# lift capacity. They have been fabulous in my little suburban properties, but they're kinda lost out in the 54 acres in PA.
You're going to have lots of folks tell you that you must have 4wd... Well, that's far less common in older machines than the new toys. It also really isn't as critical as people have come to believe. An awful lot of work has been done, for an awful long time with 2wd machines, so don't get too hung up on that, as it will really limit the machines you can pick from.
You're also going to find more gear transmissions in the older machines than hydro. Also not a deal killer, but that's more of a concern... If just gear, it will suck for significant loader work, but if it has either a mechanical, or hydraulic shuttle, then they're just fine. Again, you need to figure out your realistic use... When we were doing big projects like the long driveway, or one of the building pads, then the shuttle on the Deere was invaluable. For other projects, like cutting firewood, or when we were just grabbing a bucket of sand to stage by the mixer, then the MF with straight gears works just fine. Obviously better to have it, and not need it, but also if you really don't need it, then it makes your selection much smaller.
As with everything, older equipment means it will need maintenance. My Deere dealer has forgotten that they sold the JD300 new in 65... They only sell little tractors, and gators now, so they're no help for service... The MF place is fine, and the old Ford/NH place will work on other brands as well, so it isn't too bad... Case doesn't want to know anything about my 646's. In general, we have to do our own work. The old equipment is less complex to work on, but you might find parts support, if you need something odd, a bit more challenging.
Looking at the industrial side of the family tree will give you machines tweaked for loader use, rather than a loader frame just grafted on to an ag tractor. The JD300 is really a Deere 1020, with a heavier front axle, and beefier hydraulic pump, plus some other tweaks. Same for the 202. That's really a TO-35 (I believe) with tweaks to handle industrial use. Nothing wrong with the ag equivalents, and I'd actually like to find a 1020 to add to the fleet, but if you're focused on a loader, then looking at the yellow machines will usually get you the feature set you want for loader work.