yes and no.
google pfsense, its basically a standalone routing solution that uses an older machine with 3 nics (2 wan, 1 lan). The problem with DSL is its probably PPPoE meaning you’ll need to have each a username/password. In theory it works but in reality its a little sketchy. You can try it but I can’t promise it’ll work well. there is other linux router type distros (clarkconnect, m0n0wall, smoothwall) etc.
if you need help pm me.
all N cards should support a/b/g since they share the same signal spectrum (2.4 and 5ghz).
N is definitely worth the money IMO.