Anwar Sb,
Welcome to the forum and Moggers Community.Your name and Unimog info already came to me from upcountry.
I am glad to see someone from Karachi, willing to work with Unimog. I have 421.129 U600(L) in Ambulance Model. Dee Dee already done the brake booster conversion and it is posted under Punga project, you need to replace the Hour meter and Diesel Lift Pump with Mercedes 240D Vacuum pump available in Shairshah for PKR. 2,000~3,000. You can use Toyota small brake booster with 1" Master Cylinder (set cost around 3,500~4,000, plus 1,000 flange (plate) machine work ) for vacuum assisted brake system, however, you need to install electric fuel pump for Diesel supply.
I have done something weird which cannot be manageable by ease i.e Electric vacuum pump with lots of electronics.
Secondly, Service your Clutch Slave Cylinder, wheel cylinders or polish wheel cylinder and replace brake washers for future brake oil leakages, washers are available at Plaza Market. Remember to check you Portal Hubs and Transmission oil and Diff (Torque ) Tube Boot for Greasing before test drive. I don't suggest Power Steering, Stock Steering is reasonably good.
Oh! and last be careful with Reverse gear operation.
Please post your mog pics.
Enjoy your Unimog.