The phrase "run through it" suggests you may wish to keep the Conn 653 fully operational. If so you have a much more challenging job than adding MIDI to a gutted console. See the thread at:
viewtopic.php?f=15&t=5162In this case a Rodgers console was converted to output MIDI while maintaining some or all of its original voices. This is a job which requires above average ability as well as a keying design which is friendly to MIDI conversion. I'm not sure about the Conn 653, but with the older Conn 651 this approach would not work as the 651 keyboards switch actual audio (and pulses) as opposed to DC voltages. Since your model has a transposer I'm thinking that the keying may have been updated to where certain Syndyne products would be compatable.
You mention cost. If I misunderstood about preserving the existing functionality then you might well spend way less than $1000 to add MIDI. The other costs will vary all over the map depending on whether you buy everything you need to run Hauptwerk or use existing equipment.