TMCC (as in ERR from 3rd rail) can be put in any locomotive, functionally. The size of the loco - the shell, for example - can play a role in the smaller units.
All of these kits come with instructions; I believe that the board dimensions are available on the website. If you want sound, that will take up more room - sound board and speaker (use the unit's original speaker, if it has one).
Just take your time. The first one is always a learning experience. Stick to the basics - maybe skip the sound on this project (can be added later - it's modular).
All the existing electronics will be removed; essentially you are left with the motors. It is just putting a new "reversing unit" into the loco - but this rev unit can respond to command signals. Soldering is not typically required on a basic set-up.