The 35 is a really nice rifle. Good size, excellent handling, very accurate and smooth. IMHO these older Dianas are generally underrated, especially here in the US.

The mainspring and piston seal in the guns are of absolute top quality. Unless the seal has been abused it may well be just fine with some fresh lube as already described.

I would also mention that the screw which secures the piston seal, is secured with a tiny blind pin across the front of the piston. Removing it requires turning the screw with enough force to shear the pin. You get the impression that the makers intended the seal to last the life of the gun.

The trigger is not too hard to remove IMHO, BUT you need to study up how it works first for sure! And use a spring compressor for control, otherwise you can end up dealing with some high-speed small parts.