I have a RailKing 2-8-0 that has been modified to look like a Seaboard Air Line engine:
I shaved off the Bellpaire boiler, moved the domes and headlight, closed up 2 of the cab windows, made new running boards, and added Kadee couplers and piping.
I have a project (no photos) on the table right now where I've removed all the cast-on detail to another RK 2-8-0 boiler and extended the boiler 2". I'm in the process of obtaining/making brass parts like domes, running boards, bell, whistle, valves, headlight, etc to make it into a 4-6-0. The 4-6-0 chassis that I'm fitting it to came from an old Weaver Ten-Wheeler. Along with it I'm making a Vanderbilt tender from styrene with brass parts.
I despise it when the manufacturers make a model and then paint it for RRs that didn't own them, a good example is the MTH Hudson painted for Seaboard, they didn't own any!!!
I'd rather modify something to make it closer to a prototype (even if my efforts aren't as pretty) than leave something I know is dead wrong as is.
I'd say do as much detail work on a model as you feel comfortable doing. I have a mini-lathe and other handtools to do some work if I need to and have been fairly good at wielding a Dremel tool, Xacto knife, and jewelers files when needed 
