As stated, don't buy the cheap RS6 you're looking at, it probably has issues. Small repairs can cost plenty. I've got a fuel leak and leaking radiator now, $2k. Only buy it if you or a very trusted shop can do the labor, and buy a well sorted car from a member here if possible. Expect to be in the shop 2-6 weeks per year, $2-6k per year, that's about my average over the last 4 years.
So if you're good with the above, really like the old style and lack of tech, get one!
For a family used car, more tech, less repairs, good warranty, reliability, get the S6.
RS6 is a great car but you've got to love it, it's a marriage. BTW, does that mean Dave is Mormon