is it hard to put a DCC decoder in an IHC deisel?
No. It is relatively easy because of the plastic frame, the motor is already completely isolated. Just solder the wires as instructed in the decoder instructions, making sure to fit heat shrink tubing before soldering.
A Digitrax DH123D is a pretty good choice for the larger IHC Diesels like the C628, for any, a DZ123 is a good choice. They are about the same price. There other manufacturers that also make suitable decoders like Lenz, NCE, TCS, etc.