I too think some form of mercenary system would be a nice addition. Perhaps it'll have to wait to Civ IV, tho - I imagine it would take a while to programme.
I think you should be able to utilize mercs you don't have the techs for. That's one of the reasons people hire mercenaries - to get troops types that aren't available at home. A real-world example might be the notorious South African mercenary groups who've brought modern military hardware (and, more important, the competence required to handle the things) to various African forces suffering from poor equipment and training. Such mercenaries will bring any necessary spares and mechanics with them (and no doubt charge an arm and quite a few legs for it).