Look for an inexpensive RK or Rugged rails engine. Many times you can buy these for the same or less than just a board. Then you can sell off "donor engine" parts like smoke unit, proto couplers, motors, and the engine as a dummy unit to further reduce your costs.
This member has a few diesels for under $150 and Steam for under $175.
https://ogrforum.com/...ew-england-roads-etc
Have Fun!
Ron