Some older backhoe-loaders are just farm tractors with add-ons, whereas some others are built stronger with the backhoe/loader use in mind.
So, it depends on what you want to do. But, picking a machine that you can still get parts for is a big plus. The Ford 801 or early 4000 Industrial - for example - is pretty light. I've had a 1963 4000 close to 40 years. 172 gas engine with a Sherman aux. trans. Handy little machine - but always breaking something - including the engine block that supports the front-loader. Nice thing is - it's small and low and gets into tight places. Also, easy to get parts for since it's basically just a farm tractor. The models with the Ford 724 loader and 723 hoe are pretty good. Others - with Sherman hoes are not. Even in the Ford Industrial line sold in the 60s - there are LD and HD versions. HD versions have much stronger front axles and also engine side-frames for loader support. Many Deere machines from that time period are pretty strong - e.g. the 300 or 400 Industrial versions - which are 1020 and 2020 farm tractors that are beefed up to handle a loader and hoe. Then there's the 500 which is a 3020 farm tractor painted yellow with stuff added. I have no idea what the 600 is that you mention and I worked for several Deere dealers in the 60s, 70s, and 80s. Something new? Or maybe a 4020 painted yellow? We never sold one. If it IS a 4020, it will be a fuel hog since backhoes spend a lot of time running a low engine speeds and light loads. Big diesels do NOT do well under those conditions. A 4020 engine is pretty efficient making 70-90 horsepower - but is a dog when only working a 10-20 horse. Most small gas engines will run more efficiently than big diesels when run at light loads - and a small diesel beats them all.
Parts availablility is good for most Deeres. Case made some good ones as well, e.g. the 530CK or the 580CK. Parts are still mostly available. I've got a 1966 580CK and it's a very rugged machine. Allis Chalmers, IH and others also made loader/hoes but parts are tougher to get.
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