I have rebuilt 3 of these Kessler spindles as of now, and I can tell you I am not a fan of these units. I really hate them. I had to rebuild an 18000 RPM unit in a DMU 50 Evolution, and 2 spindles in Dmu 80P machines with a 8000 RPM ( if I recall the speed) top speed. Anyway they are a overly complicated poorly designed items. If you decide to tackle this job you will see what I mean. Please don't read this as me trying to discourage you. I totally agree that what DMG is asking for a rebuilt is robbery. So with that llittle rant off my chest let me give you what few pointers I can , at least so that you will know what you're up against.
First off I don't know from your post what the maximum RPM is, of your spindle. Usually the higher RPM units used ceramic angular contact bearings. Easy way to tell is to use an electrical OHM meter and test from anywhere on the spindle internal tool taper to machine frame. If you don't get continuity they are ceramic, otherwise they are steel.
As far as getting any type of engineering drawings, or any data on these units, I very much doubt you will find it. Kessler is very tight lipped about this. I have an old mate of mine who is a service manager for DMG here in north america, and even he has no technical info on them. So from my experience, here is what I ran into.
1. Is the spindle overheating, or is it noisy too? If it's just heat, check the spindle cooling unit behind the machine. Is it low on coolant? Another problem is the coolant pump, they tend to shear the drive keyway. Even though the motor and fan are running the pump isn't pumping.
2 Regarding the bearings. With the steel angular bearings in our 80P machines the delivery date for the bearings was 12 to 16 weeks. I did find a supplier in the USA, who had stock ,but he couldn't sell them to me as he had a contract with DMG and Kessler as to not deal with machine customers. I had to call a favour from my mate at DMG, to get them to sell the bearing to me.
3. As far as the ceramic bearings, again the led time to delivery was preposterous. These were a special type of bearing with 2 "O" ring groove on the outer races, and a .030 hole through the outer race, to feed an oil mist lube to the balls. What I ended up doing was to disassemble the bearings. This was a nerve wracking ordeal because the bearings cost upwards of 3000 dollars, and I stood a good chance of destroying them doing this nonsense, but i had to. They where never designed to be disassembled, but I had to do it, because the lads in our machine shop had to grind in the 2 oring grooves, then EDM a .030 hole. After all that and a thorough clean, I reassembled them.
4. Regarding the preload. I just put everything as it was, I found I didn't need any preload re-adjust, It was all taken up by the original spacers. No problems there, thankfully. After installing the bearings on the inner shaft, I do take the assembly across the road to another mates shop who has a balancing machine. It is important to do this step IMO for an 18000 RPM rotating mass.
I hope I didn't frighten you from having a go if your up to it. I'm just relating my experience.
I don't work for DMG now, but I did at one time when Deckel and Maho were seperate entities, so all my posts here are honest, and not biased toward the factory works.
If you need more help, please post here for all to read, and I'll see what I can do.
I just hate when the big boys, with over priced spares , try to gouge the little fella trying to make a living.