Merlins, Allison, Griffins, Junkers, and Daimler-Benz.... V-12's are all considered Inline engines in the aircraft world. The other type is the radial, (the round ones), Pratt & Whitney, Wright, BMW....
If you want a cheap Merlin, look for a Rolls Royce Meteor tank engine. Mostly the same engine, but derated for tank use. And cheap cause it's not certified for aviation use.
