You could look around for an early Accucraft IoM Railway Caledonia. It isn't battery powered but should not be too difficult a conversion. There is masses of space in the boiler and side tanks for batteries and control gear. The downside is that it is not forgiving of poorly laid track having a long and rigid wheelbase. LGB R1 curves are a def' no no.
The Playmobile diesel is an excellent option, i have one and it runs faultlessly for hours. Although I haven't got around to it I am sure it would be an easy candidate for modification.