Congratulations on a great tractor! Of course I'm a bit biased, but ours has been flawless for more than 10 years now.
A new set of 3 point arms might be available (check with Messick's), but it'll be quite expensive. An alternative might be a salvage place like this (there are others, this is just one I know of):
Kubota Tractor Parts: Engine Parts || Weaver's Compact Tractor Parts & Compact Tractor Salvage
Don't know if the 3 point arms from a "standard L" would work on a "Grand L".
Particularly if you don't know the service history of the tractor, it wouldn't be a bad idea to do a full round of new fluids and filters. These little Kubota engines are about indestructible if you keep everything clean and well lubed.
The Bradco 511 is an outstanding backhoe. I seriously considered it for my
L4610 and only went with the Kubota factory backhoe because of the dealers involved. Keep all the loader and backhoe grease points well lubed (tractor too!). It looks messy, but everything will last.
Be sure to keep the radiator clean. It tends to accumulate crud in the vanes that need to be carefully rinsed out. It should not drink anything except diesel fuel, and it's frugal with that.
For most things you do, 1800-2200 rpm is plenty. I don't like to putt around at idle and try to have at least 1500. There is always plenty of power and you should virtually never need full pto rpm. With my backhoe, 1600-1700 gives full hydraulic power but doesn't make the controls so hair-trigger that everything is jerky. Yours will probably be a bit different, of course.
I have a shop manual for the L10's, if there's any specific information you need.
