everything is available when it comes to parts for mb-tracs. i order parts for them almost every second week. mercedes still manufactures brand new parts so go for it, provided you find a good example.
oh,in case you buy a big one(1300-1500-1600-1700-1800) don't forget to take off the restrictor of the 8th gear(you aren't allowed to select it due a small piece of metal fitted at the factory on the selector cover). when you will do, you'll see 40-45mph
if you want a little more, tamper with the governon on the injection pump, the planetary of the high-low range is also different to unimog one(ie slower)
things to check
-cabin mounts( for some reason they wear them a lot faster than unimogs)
-rear diff mounts(nobody ever bothers and the whole thing moves left-right)
-the air system(a little more complex than the unimog one due to a stupid valve that connects the brake action with the three point hitch positioning)
- cabin is not the most water-tight
- heater piping goes to the top of the cab and it tends to rust
-steering pump is also wearing out faster than unimogs but it is interchangeable with the mercedes trucks pump(if it has the 110 bar, go for the 130 one)
-the crawler gear lever works with a good old fashioned cable and it tends to lose precision when selecting
- it may look that a certain example has the rotating seat but don't be fooled. the mill was there on all of the big mb-tracs, the parts needed for actually rotating the seat were an extra, so check if the seat rotates because retrofitting the rotating feature is not worth the money.
these are off the top of my head