Do you already have the battery and electronics for the conversion? I would be looking at the size of those components and figure how big a space you will need in the tender. Will you maybe need an auxiliary tender for additional battery space?
An MTH PS1 loco with a large tender and dead electronics could be a good candidate. They have a 4-pin connector between loco and tender. If you can fit your battery and electronics in the tender, you have the potential to easily swap the tender to use with other PS1 locos.
Would you be using fixed-voltage track power to help keep the battery charged? That would extend the range a lot.
