As a follow on to Al's post - When are you going to be in a position to fit this transponder?
Come April next year there's going to be a lot of used transponders on the market as people fit mode 'S'. If you want a stop gap measure that's cheap but will see you through the mode 'S' "will it / won't it happen" period do that. The market will be flooded to the extent that they'll cost peanuts (relatively speaking).