Hi Matteo,
Only you can answer which type you have to choose. But I can try to help a little:
The 65/70 was the first Trac. He was only able to run 25km/h and used a lot of parts from the small Mogs. The axles are in this case little bit weak and not recommended to be used with a top loader for heavy weights. Anyway, you see a lot of them with the small loaders, not fully hydraulic systems and they still work.
The MB Trac 700 replaced the 65/70 and is quite simillar (axles, gear box,...). But depending on production date and exact type (e.g. MB Trac 700s) they are faster and can run 32 or even 40km/h.
The MB Trac 800 has stronger axles, disk brakes an stronger gear box,.... Generelly It is a stronger design and more reliable construction. Again based on production date and the choices given by Mercedes, they can have different tops speeds.
All three types have the same engine OM314 (and the last types build 1987-1991 the OM364 ). In case you buy a smaller one the engine is limited and should show up in very long lifetime.
Generally, all three of them are reliable and good machines. Depending on the avialability, the pricing and especially the accessories you need, you should try to choose them. But the MB Trac has its weak points as every other maschine, too. Checking them carefully and discussing before buying a 30-40 year old machine is always recommended. Good luck with searching.
Best regards from Shanghai,
Matthias