To have fronts that worn I'd expect it's been used with a loader, even if the loader's not on it now. I wouldn't buy a machine without a loader personally, most people need one, and they're expensive to add later. Way better to buy a machine with one on.
A tiller should have some sort of slip clutch - because they hit stones and the like. The slip clutch should protect it from over HP as well. The tractor will only deliver as much HP as is needed to run the implement - if you have your throttle set at 540rpm, then it will deliver enough power to spin the implement at 540rpm. If the tiller's rated for 40hp, unless you're beating it through some very hard soil, it's likely it simply can't use any more than 40hp - the tractor will be throttling back to avoid spinning too fast.
Most people would buy the right size implement, because if you have 52hp, why not use it. But if you already own the tiller, or are borrowing it or renting it, it's unlikely to break it.
For some reason I recall the MX 3ph being described as Cat I/II. I thought it had both sets of bushings or something?