105 is Shimano's middle groupset and is probably best thought of as their entry race groupset. It's robust, parts are reasonably cheap to replace (especially from eBay), and it functions in the same was as Ultegra and Dura Ace. It's what I started with and I'd happily have it again.
Your gear options are probably a double (53/39) or compact (50/34). On the rear you can have various cassette options but the choice is yours depending on whether you want low spread or high.
I ride a double, but I don't think I could manage The Dragon on it (190km through the Brecon Beacons). It rocks on the flat though.
Cranks can be of various lengths nut I don't find that it makes that much difference. You get a little bit of leverage difference with longer ones but can lean lower with shorter ones. I've got 165mm ones on my fixed and 172.5mm ones on my road bike.
As for the frame, Cotic ones are supposed to be good, but why pick a cyclocross frame? Just don't want to stray too far from MTBs?