Actually the CX here is new... The fan page has only 140 fans... People confuse the cyclocross with bicicross (bmx)... But anyway...
With some friends we are doing a team... And looking for places where to practice and have some fun... Maybe start doing some races and see who will be into the CX local scene.
I was thinking to use the bike to do some road cycling... So maybe I will go with a 50-42 chainrings and a 11-32 cassette... Only use the 42 for CX and 50-42 for road... Or 50-34.....
Personally, I find a 42 too tall a gear for CX if your only ring. The 50 will be pretty useless offroad. Unless you are a super strong rider, once you get offroad, you might find the 42 holding you back. I ride a 1X set up with a 38 and 11-26 or 12-27 cassette. Rarely do I get past mid-cassette on most courses. As Chad recommended, different rings you can swap out might be your best bet. So a 50/34 or 50/36 for the road and a 42 ring to switch with the 50 for CX/offroad fun.
I have a Praxis Turn M30 172.5 crank and corresponding BB for a BB30 frame. This crank is neat bc it fits nearly every style frame/bb on the market by using the proper BB. It has 36/52 rings mounted but I could mount 36/46 rings if you'd like. I would take $200 for the crank with 36/46 rings (slightly used rings) and $50 for the BB30 bb if you need it. I also have Industry Nine i25tl Disc wheels (used since August) and a set of Roval Carbon Tubular CL 40's with Clement MXP tires (14 total rides) mounted if you need training wheels or race wheels (or both). =)
Please let me know if you have any questions and good luck in your project.
105 5700 shifters are indexed for 10 speed and will not work properly with a 9 speed RD. Running a double front setup you'll have no chain drop issues using a 105 RD, so I would suggest that instead of the SLX or XT.
If you're riding offroad with this bike, go 46-36, or if you're more powerful maybe 48-36.
If you're going to be on harder gravel and pavement, the 50-34 should do fine.
I've been using an Ultegra 11-speed setup but with a single SRAM 42t in the front (and therefore no front derailleur) and 11-32 in the back, and it's perfect for anything I'm doing with my cross bike. I'll never need more power than 42x11 can give me, and with the SRAM narrow-wide chainring, I haven't dropped a chain yet even when riding through sloppy mud.
I have been very happy with my budget 1x9 groupset. I use a top of the range KMC 10spd chain (the gold one) on a wide range 9spd Tiagra cassette, with a XT 9spd small cage rear derraillier and Tiagra shifters. At the front I use a Narrow wide chainride specifically for 10spd chains and a pair of second hand ultegra compact cranks.
This has worked so well, it is so tough and has not put a foot wrong in 1000s of bumpy miles. The two drawbacks are the relatively large spaces between gears and the fact that you have to run an inline cable adjuster. Top tip for this set up is keep the cable tension high, the strong spring in the rear mech allows for this and keeps changes positive and crisp.