[Rimrock]

Would advice Servonaut SM3 (easy 'plugg and play', 3-4 different engine sounds), or Beier (more complicated, but higher number of engine souds to choose from) if you already familiar with that unit.
The Servonaut SMB got in addition to engine sound (just 1 off, Liebherr), sounds for tracks and more. Coll when things are set up as supposed. All is set up on a certain number of mixing channels, one need a radio with quite a few free mixing opporturinties. It's adviced to read the online manual before going for SMB, and compare what opporturities one got at the radio one like to use. To fully use the SMB possible might be a challange with the original Turnigy/flysky 9 ch radio.
Type of speaker ohm, at least for servonaut modules, varies depending on what voltage and type of unit. It's explained on the servonaut site what goes where.