The 445 and 455 with a 40 loader are both very good machines, and are good for several thousands of hours if properly maintained. I had a 425 (carbureted) with an hour meter reading over 3000, and it still ran just fine.
The 445 had the fuel injected Kawasaki v-twin, which is a very reliable motor, with only a couple of weak spots. The most notorious is the nylon camshaft gear used on the pre-2000 (I think) units, which tends to fail at around 1000 hours. It's not too bad to replace, assuming that nothing else got damaged when it let go. Another common failure is water pumps leaking, which isn't too bad either, and plastic valve covers leaking. There are replacement metal valve covers available from Deere. If you find a late one, or one that has had the cam gear replaced, it should be good for a very long service life.
As far as I know, the Yanmar diesel really doesn't have any weak spots, other than maybe glow plugs and the injector pump solenoid. I think both are easy fixes. I don't have a diesel, so I'm not quite as familiar with them, I just know that I want one!
Typical 4x5 issues include broken plastic hoods and side panels, which are expensive, and necessary for proper engine cooling. The plastic gets brittle with age, and doesn't repair well. The earlier style (pre-99?) steering system is also a bit wonky, with more moving parts that wear out than is really necessary. It was used because it was the same parts as the 4 wheel steer tractors used. The later style only fit 2 wheel steer tractors, but in my opinion is a better setup. I bolted in a front axle from a late one on my second 445 ('94) that uses the simpler steering system. Transmission leaks also pop up, typically the diff lock seal or the swash plate o-rings, both can be fixed for about $3 in parts, but will take several hours of muttering about engineers needing to work as mechanics first...
Hooking up the deck driveshaft isn't all that easy if you're not flexible any more, but once you get used to it, and keep it lubed up good, it's not so bad. The 60 inch deck is a beast of an attachment, and is very heavy to move around, almost twice the weight of the 54 inch deck. Both decks are very good and leave a good cut. The 48 inch deck cuts well, but is no good for trimming around obstacles, as it is the same width as the tires. 4 wheel steering can be nice for some mowing situations, but is not so good with a loader. I'm not even sure if the loader will fit on a 4 wheel steer tractor. The loader goes on and off in about 3-5 minutes. I made this quick video (with my very dirty tractor) for another forum member to show the process
No matter which you buy, if you're using a loader, you'll need ballast. I'd say minimum for mulching on flat ground would be loaded rear tires, and 6 suitcase weights somewhere off the back. I now also run triple wheel weights, but it's a dedicated loader, and I have a lot of hills.
It's typically easiest and cheapest to buy as a package with everything you want (or more, and sell what you don't) than to buy pieces at a time. Finding just a loader is pretty rare, and when they come up, they usually aren't cheap. Of course, as you fall further into the world of garden tractors, you'll want a second one, so that you can leave the loader on all the time, and have another for mowing...