I'm not US based (don't use that pricing tool), and don't personally have those options. So take everything I say with a fair pinch of salt.
The rear remote hydraulic valves are for rear implements. You have the choice of 1, 2 or 3 pairs of valves. 1 pair of valves lets you run one "circuit" - one valve is in and one out. So you can choose to run 1, 2 or 3 functions on the rear. If for example you were running a rear blade, you might have a tilt and an angle, which would be 2 functions.
For the 2nd and 3rd circuits, you then get to choose whether or not to have float. I'll talk about float with reference to a front end loader, which is where I'm familiar with it. On a FEL you can have your stick in the middle - which is locked - the loader stays where it is. If you have your loader on the ground and drive forward into a rise, it'll dig into that rise as the loader can't rise over it. You can push forward on the stick which lowers the loader (positive pressure down - once it hits the ground it'll start to lift your wheels off the ground if you don't stop pushing forward), or pull back, which lifts the loader up until it hits the top. If you push the lever all the way forward, it clicks into place (detent), and what it does is basically disconnect the hydraulics and let things "float", it lets the loader drift down till it hits the ground, and as you drive forward your loader will contour follow. That's useful for example for scraping snow off your driveway without digging big chunks out of your paving - you curl your bucket very slightly back so it doesn't dig in (about half an inch or less up at the front), and set the bucket to float up/down. It'll scrape along the driveway without biting in and without you having to adjust it.
As for what you'd use float for on a rear remote, I have no idea. Perhaps there's some use in your top link floating to somehow terrain follow with a box blade? I'd guess that you need to know what implements you're putting on before you know whether you need it. If it doesn't cost much more, then I guess I'd be inclined to get it just in case. Looks to me like the float version costs $42 more....so depends whether that's a lot of money for you.
Also looks like the first valve costs about $700, the second and third about $600 each. Again, depends on how tight money is for you, but if you don't know what you'd use them for then I'd question whether you need more than one.
On the loader, the heavy duty loader with 2 lever style coupler I think is the skid steer quick attach.
I can't see any option for a third function kit. A quick google doesn't show up a Kubota one, but there's a Land Pride 3rd function kit for the L2501. I'd guess that means it's not available as a Kubota option, it's after market. Of course Land Pride is now owned by Kubota, so it's still all in the family. The BX only just got a factory 3rd function option, I think the B might not have one, so it wouldn't surprise me if the L2501 didn't have one. So you'll have to talk to your dealer about one if you want it fitted. Looks like it costs $730 from Messicks:
https://www.messicks.com/part/380-152a/kubota-std-l-series-third-function-valve-kit. There'd be installation costs as well I'd expect.
For the guy who's looking at a grapple and forks....you don't need rear remotes at all for that, you need a 3rd function kit. For a bunch of this stuff you're really better talking to your chosen dealer, because they'll know exactly what you need, they'll explain it, and to be honest will probably talk you out of some of the things people on here will tell you you need. They'll be able to better understand what you're trying to do, and show you how it works on the tractors that are there - it'll all make much more sense then.
