But you may well be right about using something other than a Turbo on a North Downs route diagram, as it and the Basingstoke service are fairly self-contained and you could presumably limit the number of crews who would need training to work say a 158, if they can hire one. Certainly can't see any lone stand-in set being let loose on the main line.
Believe the majority of crews working the North Downs line already sign 150s for the Basingstoke services, so using 150s may be an option. Southern drivers only sign 165/6 AFAIK so they'd have to be trained up on any alternative traction.