I had the N66U and it's a great router but I recommend you get the AC68U now. It has a dual core CPU so handles loads much better and also the wifi is AC so at least in theory you can get the full 200 Mb (and 300 Mb should it go to that next year) over wifi, which is not possible with the N66U.I'd say about £100ish tops. Just looking at the Asus RT-N66U which seems to get good reviews but not sure if there's something better out there.
I have both the SH2 and SH3. I have them both in cable modem mode but have to say I prefer the 2 as with that I could dim the lights. The feature is no longer available in the SH3 and I don't need it's AC wifi as it's in modem mode.