Cisco Firewall :: 5505 High Availability Over Dual WAN Connections

One of my remote sites acquires Internet connectivity via a cable  modem service.  This goes down intermittently, of course.  I would like  to purchase DSL service from the local telco and configure the edge ASA  (currently a 5505) to use the cable modem path normally ... and fall  back to the DSL path if necessary.
 
These seems hard to  do.  The edge box would need to evaluate the viability of a WAN path  using some set of tests ... perhaps pings to a handful of major Internet  sites.  If all those pings start failing, it would stall for a minute,  to give the WAN service provider time to recover ... then cut over to  the second path.  Cutting to the second path might mean pushing new DNS  server addresses to clients (or perhaps the edge box would hand out both  sets of DNS servers all the time and rely on the clients to try them  all.)  Once the cable modem provider restored service, the edge box  would stall for a while (ten minutes?  an hour?) and then cut back.
 
I'm willing to replace the edge box with something  fancier (a bigger ASA or something sold as a router or whatever),  although I'd like to stay under 10K (list) for such a replacement.