I'd suggest at least trying the present amps / speakers. Copeman Hart equipment is well made, and they may well be fine. If they are too old and sound dreadful, you'll at least have tried to save some money! I'm assuming that as you say there are 4 stereo pairs of speakers that there are 4 stereo amps (=8 channels). Equally, they may be OK for now, but part of a future upgrade.
You'll need the Advanced edition of Hauptwerk for multi-channel output, of course (you'd probably want the voicing capabilities anyway) and a decent multi-output soundcard. The M-Audio Delta 1010LT gives good results and is excellent value for money. Then you can reduce the "deafening" problem you describe by routing ranks as you choose. C-H usually include a good subwoofer on the pedal channel(s), so it would be best to use their ped channel for ped, wherever the speakers are.
You could use a stereo pair and subwoofer as the minimum, but even in a small church I think this would be disappointing. By using more speakers in a multi-channel setup, you get a more realistic spread of sound and less IM distortion. "Relatively small" speakers would need suitable subs for at least the 16' stops, though, again, careful routing can reduce the need for these on some channels. David Pinnegar has made interesting use of large numbers of low-power speakers - see his various posts.