Southpaw -
Not sure there's any "external keypad" bluetooth kit, but rather than a cheap quality "plug in" kit, why not source a Nokia bluetooth kit or similar (I have a Parrott CK3000 that cost just £59).
Whole thing goes behind the dash. Its dead easy to wire in.
You just connect one wire to a live feed and one to an ignition feed on the fusebox at the end of the dash.
You connect the two speaker wires into the "Tel +" and "Tel -" connections on the back of the radio and the Mute wire to the "Tel Mute" connection on the radio.
You then have a full "built-in" bluetooth kit that mutes the radio automatically and plays through the car speakers just like the really expensive factory setup.
If all you want to do is answer calls whilst driving, then just leave it like that and leave your phone in your pocket....
Otherwise, you just find a neat way to mount your phone on the dash - I have a small and neat suction pad unit that sits on the corner of the dash and the phone just holds on to it by magnetism, pulling off easily when I get out of the car....