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
I think used or rebuilt models of the vacuums mention are great, long-lasting buys if given proper care. However, my only concern is that the buyer educates his or herself as regards purchasing them through eBay. Most of the local indie vac shops in my area seldom have these cleaners available for sale.
Online eBay shoppers should always check out seller feedback and shipping prices.