The Airstream uses a really long sock type HEPA filter (over 15 inches long total and filter portion is 12.5 inches long. It is 4.75 inches wide). The last time that I bought one they were about $28 at AirWare America and seem to last at least a couple years. In the seven years that I have owned mine, I have changed the filter twice. I checked the airflow earlier this week and it is still very strong. The HEPA filter can't be cleaned without damaging its filtration capability.
The "prefilter" is nothing more than a piece of reticulated foam meant to catch large particles and small bugs. Any dust that it captures is purely coincidental. I don't know what they cost, but all that you need to do is rinse it under running water occasionally. One prefilter should last ten years with reasonable care.
The Ni-Cad battery pack is exceptionally well made and has a very high capacity -- they say about nine hours per charge and I would concur that seems to be my experience. I am still using the original battery pack from seven years ago. Back then, replacement batteries were about $150, but they are now about $250 -- ouch. Why are they so expensive -- they have to comply with all sorts of industry standards for intrinsic safety since they are used in environments such s coal mines and other dusty work locations. In order for the battery to be considered intrinsically safe means that it has been certified to not get dangerously hot if short circuited, not emit any harmful or explosive gasses under any operating or storage conditions, not have any exposed arcing contacts, and not burn a hole in your wallet (oops, looks like it failed that last test).
It would not be nearly as convenient, but for woodturning, a frugal person could build his own Li-Ion or Ni-Cad battery pack (4.8 volts) assuming that batteries of the same current rating and A-Hr rating could be located for a reasonable price. By the time that you rigged up your own battery pack, made some contrivance to hang it from your belt, added a on-off switch, an electrical connector to match the one from the helmet, and a charger, you might have spent a tidy sum on a battery system that only has half the capacity of the one-pound battery that hangs from your belt.
I have replaced the Tyvec face shroud once and it is about time to replace it again as the one that is on the helmet is looking quite grungy. It is also about time to replace the foam temple seals and perhaps the sweat band -- these have never been replaced.
I would suggest getting the anti-static face shield. They only cost a couple more bucks than the regular face shield, but during dry weather, electrostatically charged dust likes to stick to plastic -- especially clear visors. Don't ask how I know this, but wiping a dusty face shield with your hand while turning is not a good idea -- it will turn a slightly dusty shield into a completely obscured one. The plastic overlays may or may not be a good idea -- they are cheap, but for some reason dust likes to get between the shield and overlay. Whatever the adhesive used on the overlays is, I have not found anything yet tht removes the sticky residue.
