Welcome to the forum Cliford.
I have converted many locos, both diesels and steam engines. In my opinion, steamers are the simplest if they are DCC ready because you can put the module and a speaker in the tender with little fuss. For diesels, the easiest conversion is one with speaker mounts already in place. Atlas has some, ad does Athearn. Unfortunately, it is hard to tell which ones do before you buy them.
DCC ready locos are the easiest. It's mostly plug and play plus two speaker wires to solder. If they are not DCC ready you will have to hard wire them, which is a little more involved.