Especially the Finns on this forum helped me build it, and sometimes I was a bit afraid of asking AGAIN some technical questions.
On the other hand, I hate to invent the wheel myself, again, and thats why it is nice to have people who can help and already done such a thing.
So to prevent, that other people begin asking the same questions that I asked, here is a sumup of the things that i did to have a OM603 turbo in my w201 (2.3-16 originally)
engine: w124 300 turbo
starter: w124 300 turbo
flywheel and clutch: w201 2.3-16
getrag dogleg manual gearbox: w201 2.3-16
aluminum engine mounts which are bolted to the engine: w124 300turbo
rubber engine mounts: w201 2.3-16
rear gearbox mount and support: w201 2.3-16
cardan axle: w201 2.3-16, but shortened by 40 mm on a turning lathe
The 3 shift bars (2.3-16) I shortened also 40 mm.
The steel plate/support which is under de gearbox rubber, is also from the 2.3-16, but the holes for the bolts that go in the chassis, are sleeve-holes now (round file)
tacho-cable can be used from a 2.3-16 also, and the clutch hose is also re-usable from the 2.3-16.
On the front of the engine: Use a steering pump from a w211 e230 or e240 gasoline car, otherwise you cannot fit a big radiator.
radiator I took is from a w124 300D NON turbo.
w124 Turbo radiator is to wide IMHO
On the top the radiator can be clamped to the original w201 chassis part. On the bottom you have to make a custom steel support system for the radiator.
Cut of the original DOWNSIDE radiator mounts of the w201 chassis, you cannot use them.
On the custom steel support, i also mounted the w124 300 turbo Oil cooler, on the drivers side, behind the bumper, on the same place where it is in a w124.
Water hoses: DIY
I used different hoses from the junkyard, cut them, shortened them, connected them (with peices of tube) in another angle etc.
I also let a friend of mine who is a plumber, bend a steel water tube, which goes from the dash-heater to a pipe beneath the turbo.
Rear diff: original 2.3-16 is WAY too short.
I used the 2,65 from the w124 300 turbo, but changed the rear cover and 3-claw to use it in the w201 chassis.
At the moment my car is mounted on 205-60-r15 tires (originally 55 height).
For track use it would be nice, but yesterday on the autobahn, I constantly felt like shifting to 5th gear, when it already WAS in 5th.
So maybe its better to search for a diff. from a w124 400E. I read somewhere that that one is about 2,42, so a bit more overdrive than 2,65. I didnt measure top speed yet, because my tacho wasnt working, but I dont think you will reach 200 KMph without overrevving with the 2,65 and getrag.
Automatic box is overdrive in top gear (and therefore better for economy and highway use), but for track use not ideal i think.
Also you can mount bigger wheels and tires (17's 18'maybe) to make up for the short drive-ratio.
Anti roll bar on the front: w124 300TDT, which is 28 mm thick.
On both sides cut off about 1,5 cm.
Steel plate mounts for the AR-bar: I cut the w124ones in half, and the w201 ones, and used the TOP sides (chassis) of the w201, and the BELOW sides (AR-bar) of the w124, and welded them together.
I used a bilstein spring&damper set for a w201 2.5turbo or 2,6 gasoline, but cannot tell much about handling yet, because my car has to be checked for steering geometry.
You can lower the weight of the front bumper drastically (underneath: impact-foam, and alu bumper bar) for a bit compensation of the heavy om603, but when you hit something: bye bye front bumper.
For the rest: vacuum system, electronics (not much), steering hoses, fuel hoses: DIY, DIY, DIY
It is nice to buy a complete DONOR w124, because you need some other parts also, and that way it would be cheaper than buying everything apart.
If I left anything behind, or you wanna know something: feel free to ask.
I'll see I I can place some pictures in the future.
Driving: I think the car isnt much slower than with the original 2.3-16 engine, and I am talking STANDARD turbo, STANDARD injection pump and NO intercooler (first upgrade), but very cheap fuel

In a few days I can tell a bit more about performance, by comparing to a friend's car.