Mike,
Good question, and let me preface my answer with "I don't have a clue".
I suppose in a perfect world I would like to go with the second of your two options, but that presupposes being able to get an impellor (don't know where from), a sufficiently powerful motor ( thinking 3-5hp from what I've read online) and perhaps more importantly getting the whole thing to fit together properly.
I spoke to Axminster about just getting the impellor/motor from their ADE4000 extractor and it didn't work out to be cost efficient at all (better buying a whole machine). Also this seems to be the only single phase machine with sufficient power and a 6 inch intake that I've been able to find.
I suppose what I will end up doing is buying an ADE 4000 and adding 1 or two fine filters (such as the one on ebay you linked to).
If anyone has any alternative solutions I'd be very grateful to hear them.
Thanks