how about a nice rebuilt TriStar (CXL or DXL/EX-20) they have great filtration; nothing gets past a Tristar (when HEPA motor filter is installed) They're also made of die cast aluminum & built like a military tank, can last forever.
2-speed CXL (best Tristar vacuum ever made) on eBay
http://cgi.ebay.com/TRI-STAR-CANISTER-VACUUM-CXL-SAVE-BIG_W0QQitemZ130360656744QQcmdZViewItemQQptZUS_Vacuum?hash=item1e5a19c368
Micro-filtration paper bags for Tristar
http://cgi.ebay.com/48-Allergy-Bags-for-TriStar-Tri-Star-Compact-Vacuum_W0QQitemZ270496033672QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3efad1eb88
or a Filter Queen
http://cgi.ebay.com/AWESOME-FILTER-QUEEN-MAJESTIC-VACUUM-CLEANER-W-WARRANTY_W0QQitemZ400098941735QQcmdZViewItemQQptZUS_Vacuum?hash=item5d27c15b27
or Electrolux 2100 on eBay
http://cgi.ebay.com/ELECTROLUX-AERUS-2100-CANISTER-VACUUM-CLEANER-W-TOOLS_W0QQitemZ380199785772QQcmdZViewItemQQptZUS_Vacuum?hash=item5885ac552c