I'd say go for it. Apart from dabbling with a loco in my teens, and a part built hit 'n' miss engine, these are my first engine build:
I've made everything, apart from the steel boilers and rubber tyres. Also produced a lot scrap, nearly enough for a third engine! Like Jason said I started at the front end, but moved onto the gears qjuite quickly, because I like gears. At any given time I make what interests me, rather than doing things in a logical order.
If I can do it I am sure you can!
Personally I wouldn't buy a part built engine, as one is never sure of the workmanship and what changes may, or may not, have been made. I'd rather mess it up myself than buy someone else's mess up.
Andrew