Diesel starter is an easy affair, 2 bolts plus electrical connections and wrestle it out. Believe the bolt head is 3/4" compatable so you can use a standard socket.
To clear up the confusion Bosch never made a gear reduction starter for the D24 diesel, Hitachi made the starter and was rated at 2.1Kw used ball and roller bearings and did not require a support snout on the end making it easier to change. Its a snappy little motor but doesn't tolerate long crank times, its also a bit more efficient than the old Bosch motor that used bushings and tape wrapped coils. Hitachi is a sealed encapsulated part. The picture shown on the website is NOT the Hitachi gear reduction starter, its a generic Bosch for the B2x gasoline motors. The hitachi has a defined offset look to it along with big mounting ears and lots of space between the motor and solinoid itself. You can fit a finger between them.

Problems you might encounter with getting the wrong diesel starter; bolt length issues, I dont remember if they were diffrent lengths or not but its easily solved as the bolt is a 12MM shank 1.75 thread known as M12X1.75 and the battery positive cable lug end will need to be enlarged slightly with a reamer to fit the larger lug on the motor. This ONLY applies if you have the older style Bosch motor and not the Hitachi, after 1985 all 760 diesels were "supposed" to have the hitachi motor and all the ones Ive seen had them.

If the mechanic claims to have removed the starter and rebuilt it you will find wrench marks on the bolt heads regardless of rust, these will have a contrasting look to virgin rust. Both of my diesels have the hitachi starters and none of the bolts have much if any rust on them, even if he did get the thing out without disturbing the rust you will see rub and small scrape marks around the bolts not to mention greasy fingerprints on battery cables and other places. Its obvious when a part is changed on an engine no matter how careful one is.
Before you shotgun the starter do follow the advice given by the other replies regarding the ground connections, wiring and battery condition. With a starter taking say 2 kilowatt that comes out to 250 amps at 8.0 volts cranking. Any slight resistance will slow things down in a hurry.

To address soldering brushes, do it at your own risk, alot of current passes the junction and along with heat enough to soften the solder. The proper repair is spot weld the flyleads to the copper bus bar using phospher-bronze based silver solder.

The leak you are seeing is the coolant inlet to the engine, someone overtightened the hose and crush cut the rubber. You can trim that back to the cut part using a razor then put the hose back on and fill the engine back up. If your inclined to deal with the arrogant people at Borton be my guest, they see the coffee bar as being more important than helping customers and it makes a great diversion. Wont deal with Borton at all. That lower hose is unique to the D24 engine however one could get creative using 1 1/2 inch copper pipe and soldering elbows to mimmic the hose and use rubber hose itself to couple things together. This however is limited to creative thinking and is outside "the box".

Are you in the Minneapolis/ST.Paul area by chance? Have proper D24 tools and can travel some.

Brandon 988