Klaus Leimbach
http://www.leimbach-modellbau.de/ built the hydraulic systems & miscellaneous other parts and Harald Stahl
http://www.stahl-modellbau.de/udp-005.html who milled the bodies, those two guys started together back in the 80's to produce RTR models for the average rc hobbyist unable to do so for himself. Harald Stahl died of a blood disorder @age 60 in Spring 2009, his son Lars took over & ran his father's side of the business into the ground by the Fall of 2011!
Klaus Leimbach also builds hydraulics for Wedico since early 2000's, and as well as for the general population that wants hydraulic components for their own scratchbuilds... so despite his partner's kid running half the business to bankruptcy, Leimbach carries on doing his own thing with his son. I don't know, nor have I heard if Leimbach continues to make any of the Stahl body kits.
Vario Earth-Movers
http://www.vario-earthmovers.com/ is a subsiduary co of Vario Helicopters. Frank Preisendoerfer is an employee of Vario Helicopters, who had an interest in constr equip modeling too and the JD850 I have is his creation that the company he works for produced & marketed the model for him. (I don't recall exactly what yr it came out 08 or 09, I got mine in the Fall of '10 and built it over the winter)
My JD850 is 14th scale, but with a simple body change (provided by Vario product option) - it can be a 12scale JD450. John Deere track hoes sold in North America are actually made by Hitachi ---> different color & graphics. Hitachi 870 in 14th scale, Hitachi 470 in 12th scale = 4 different choices in one model. Pretty clever, eh?!
Since the Vario track hoe, Frank has established his own model company (and still works for Vario Helicopters too) called
http://www.fumotec.de/ with a Bell 50 & Bell 40 ADT, Bell 4206 4x4 tractor, Komatsu WA500-6 wheel loader, Ponsee timber harvestor and soon to be released Komatsu PC 490 track hoe.
Per your order quote?
14mm bore would be much better power without sacrificing too much in flow, maybe even 16mm, considering the difference in weight of 2mm aluminum vs 18 guage steel. 145psi (10bar factory setting) -> in my little Stahl excavators with 14mm cyls I turned my press up to 15 - 16bar (over 200psi) without any harm in the short term (couple hundred hrs use).
Be able to dig hard ground with your home brew 320?... not likely, not really. I guess it depends on what type of ground you're referring to?
My JD850 with a gvw of 80lbs, 21mm bores with a working press over 400psi doesn't dig hard ground by the bucket full with each stroke... more like it peels off 1/4" slivers at a time in moist virgin clay loam soil. A ripper helps alot, but I don't rip very deep in one pass, it takes several to get that 4 inch tooth sunk to ground level. Well, you see how it is in my videos with it.