The Massey 135 is indeed a fine tractor and at that price, one should expect some work needing to be done. The nice thing about the 135 is that full shop manuals are available on Ebay for about $5 (UK version) and while a bit tedious to read, they cover everything.
I would venture to guess your brakes need adjusting, are oil soaked or the shoes are glazed up from driving around in high gear and heating them up on hot stops. Glazed shoes can be roughed up with sandpaper and will be fine. If oil soaked, either clean them up or replace them, but replace the seals to stop the oil.
Complete injection pipe sets are available for the MF135, but try reflaring the ends first if not cracked. Lines are copper and can be often salvaged.
Wouldn't worry about the cake on the engine, diesels can tend to pick up a lot of dirt from dripping fuel, leaking rocker cover gaskets, spills, etc. Pressure wash the whole tractor so you can see what you have. Best to remove the cowl so you can get the top of the Perkins. Remove the rear bolts on the side, loosen the two below the radiator and tilt the entire thing forward and off. Best to use two people. Be mindful of the lights wiring, there should be slip joints for unplugging them. If not, nows a good time to add them.
The 135 is a tough tractor, parts are plentiful and cheap and the Perkins diesel is an excellent engine. I would say any 135 diesel running well is well worth the price you paid for it.
I wouldn't worry about the issues you are dealing with at the moment, they are common to all tractors (unless new).
Attached is a picture of my 135, currently being overhauled for backhoe duty.