I agree that finding a used FEL is a real challenge. That's because they're so incredibly useful that once people get them mounted on their tractors, they never take them off. That's good news, because it means that even though you have to pay for new, it's a tool you'll get a lifetime of use out of.
You do have the option of looking at aftermarket FEL's, but since you have a current model tractor, I'd generally recommend sticking with a loader designed specifically for your tractor. I don't own a Kubota, but I think there are two loader choices for the MX5000 -- standard duty LB702 with about 1800 lbs. lift capacity and heavy duty LA852 with about 2400 lbs lift capacity. In all honesty, you'd probably be fine with the LB702, but if the price difference isn't too steep the LA852 would never leave you wondering if bigger would've been better. The only tradeoff in going to higher lift capacity is that the larger hydraulic cylinders will move a little more slowly -- you trade off speed for lift capacity. It's worth trying one on the lot to see how the loader you want operates before you put your money down.
There may also be bucket options for each of those loaders -- standard duty vs. heavy duty, as well as different widths such as 60", 72", or 84". For the size and horsepower of your MX5000, I think you'll like a 72" bucket. Having a bucket just a touch wider than your tractor is useful in all kinds of situations. As for standard duty vs. heavy dutybucket, again you'd probably be fine with standard duty. If you're likely to worry after the purchase if you made the right choice, then spring for the heavy duty bucket up front.
You may also have options for quick-attach -- both for the loader itself (where it mounts to the tractor) and/or for the bucket (where it attaches to the loader arms). Personally, I have never taken a loader off of a tractor -- I find it always useful to have it mounted. I do, however, like to be able to remove the bucket quickly (travels easily to the welding shop that way), and one day you may want to swap the bucket for forks quickly.
Lastly, I'm sure your dealer will recommend two additions to your tractor to go with your new loader: 1) a grill/hood guard for the front of your tractor and 2) fluid ballast for your rear tires, if you don't already have them filled. Make sure your price quotes include both of these -- they're indespensible.
For the last item to consider, some people swear by 4-in-1 buckets. I've never had one (a little pricey for me), but I can see how they could be useful. If you've got the extra dough, you may want to consider one. I'm sure your dealer could swap the Kubota bucket for a Long or other manufacturer's 4-in-1 for the cost difference at the time of purchase.
There are other posts here about mounting chain hooks to your bucket -- you'll certainly find them handy if you go that route. Your dealer may even install/weld them on and paint them for you if you ask for them as part of the deal. I drilled a hole and bolted a chain hook on mine for about $8.
Last of all, if you've been needing some gravel, top soil, or other material around your property, plan to have it delivered and dumped for you to play with when you get home. It makes for great practice, and just a little bit of fun.
Parker
