[suicideneil]

Welcome, you came to the right place.

Since you already have the nitro chassis it makes more sense to finish it off than to buy all the Erevo parts (chassis, tranny, skids etc).

MM esc works great with 4s lipo and the 9XL, though lots of new motors are appearing and more are planned in the following months, so better choices to be had for your budget will soon be available. Just use a small fan and an external BEC (Battery Eliminator Circuit) or Rx pack- remove the redwire from the Rx lead.

TeknoRc vs RCM motor mount is a good question, but they are 2 different ways are doing the conversion. The tekno uses a clutch so that you can only run in forwards, and use mechanical brakes. The RCM route allows you to use forwards/brake/reverse via the motor and esc if you lock the tranny into 2nd gear, or keep it unlocked and use the esc in forwards only mode, to utilize mech brakes. Either route works fine, but if using the MM the RCM option offers more possible setups.

Only real thing to concider is the need to buy or make your own battery and electronics mounts with the nitro chassis, but there are plenty of existing conversions to look at for inspiration in that department.