I'll second what has been suggested about staying away from the 400/500 series. They are nothing more than a little bigger utility tractor, not up to the rigors of daily use as a loader tractor. I'd be taking a serious look at a 5/6000 Massey. Better built than your existing Deere, and available with plenty of power to turn any round baler you might have.
As far as weak hydraulics, I'm not sure what anyone means when they say that. Massey has always had excellent hydraulics, in so far as the pumps were concerned. You'd occasionally see a bad priority valve, or a leaking line, but thats about it. Heck, a popular upgrade to the 4000 series Deeres that everyone loves so much was to put a Massey radial hydraulic pump in them.
On the Kubota side, they're not a bad tractor, but the 120HP version has rear end and PTO trouble (this from a mechanic friend of mine who works for Kubota).
Another thing to keep an eye out for is a 7110 or 7120 Magnum. They are a little older than what you're currently looking for, however if you can find one with a couple thousand hours on it, its a keeper. They are simply the BEST front wheel assist tractors ever built.
Rod