I would absolutely go for the Kubota. There is no issue whatsoever with getting parts for these. There is no reason to think that a 15 year old tractor is old and worn out, or that it doesn't have "enough" use on it.
The tractor looks like it's in great condition. Did you get any photos of the cab? Does it have rear remotes?
I have seen 2 isolated instances where the toplink connection point at the tractor end, has been pulled out/broken off by extremely heavy loads bouncing over large bumps and jarring the casting off the tractor. This is a VERY expensive fix, but also VERY unlikely.
If the rears are not loaded, you will want them loaded right away. I made the mistake of waiting, and after using the FEL a few times realized I absolutely needed the wheels loaded.
I use a 1300lb box blade, a 1,000lb rotary cutter, a 1,400lb
chipper, a 1200lb stumpgrinder, and other numerous (lighter) attachments with mine. I've never had an issue with it.
If you want to add rear, or front hydraulics, there is a step by step thread in my signature for doing so to this tractor. There are not many of these models as it was only built for two years.
For the uses you describe, it will NOT be too big for you. I was originally looking for the smaller L series, and posted on TBN and was talked into going with a larger L series. I'm glad I did, as I actually feel like this tractor is still too small for what I want to do.
Look at it like this, aside from shinier plastic, HST +, more modern looks, a fatter wallet, and bragging rights, this tractor is identical in capacity to the new grand L series regarding FEL lift and power. The price is good, but as mentioned offer lower. I think I paid 16,500 for my open station version about 5 or 6 years ago???
If I wasn't looking for a bigger tractor, I would very likely buy this one you posted about, transfer over all my hydraulics, and sell mine, just to get the cab and fresh tread on the tires!
One thing to consider if you have multiple buckets or will purchase a lot of used skidsteer implements, is this tractor most likely has Kubota's pin on quick attach system that is not compatible with the skidsteer quick disconnect.
Go back and forth between my bucket, grapple, and pallet forks (actually almost never use those anymore) with the pin on system, and it's not nearly as easy to line up and use as the skidsteer style. I've thought about converting it, but I'm likely selling mine in the spring anyways.
Another thing to consider, is that if you are doing heavy forestry work, the loader on these is not very robust. I've managed to not bend mine by being mindful of my loads, but it does torque and twist with heavy use, as do all their compacts in this range. I imagine that the LA852 will be stronger than the smaller model loaders you are considering but I don't have direct experience with those.
For my uses, I find the
L4610 a little too small, lacking a bit of power at the PTO, and a little too 'delicate' for what I do. It all depends on your uses, but I would ABSOLUTELY NOT GO ANY SMALLER!
Just my humble opinion

. Feel free to PM with any specific questions.
(If you decide not to buy this, please share the contact info with me just for the heck of it
The
L4610 is a pretty capable tractor for it's size. Look here for more pics of what I've been able to do with mine....
http://www.tractorbynet.com/forums/attachments/211635-grappling-fun-picture-thread.html
