Hi Woody, so sorry to hear as to you having been diagnosed with emphysema. Now to see if we can keep you turning out that wonderful work.
The specs looks good for the MC420 it is ideal for a workplace up to 40 cubic metres in size (single car garage) a little smaller than your 20' x 10' then you just want to use it local to the lathe, it is compact 250mm high, 204mm diameter and very ecomical to run (100watt motor), yet it will filter that 40 cubic metre space 10 times each hour, as that what does your space work out as.
This new version MC420 has replaced the MC400 and differs in it draws air from around the side and from the bottom of the machine and has two filters, one that wraps around the side (as 400) and one now in the bottom. I have the 400 and it will keep the dust down as recommended making it ideal for over a lathe, as for noisy no more than any motor or fan, I'd say about the same as the 1st stage of a cooker hood, but not noticed over other machines.
My shop is now 12' x 10' so it's more of a draft from it, which the wife when on a lesson dislikes.
It would work well with the Jet you have already. Next what else do use chip or dust vac, I've the Yorkclean WV100, what I'd like to do and this is always recommended is duct the dust away outside, have you such a ducting system, it would be ideal, as I find in using the vac off any machine port you never catch all the dust, bandsaw and sander in partially, it really need extra pickups at the table or under it to catch everything, should you be able to get the dust outside and use the ambient filters during working and can run them still after you finish and not just when you stop a machine the only contact with dust would be to empty/service the vac and short term for that you could then use your trend or the power cap without hurting your neck to much, this all sounds fine to say and I hope it has been of help, the last thing we want is not seeing you turning.
Cheers David