Upgrading the brakes on a Giant Anyroad 2 2015 I am looking to buy some used Giant Anyroad 2 2015. The groupset is already upgraded to Shimano 105 2x11, with cassette 11-34t. Crankset are Tiagra. Everything look fine, except the brake, which still using SRAM Avid BB5. Based on some review, SRAM Avid BB5 is having bad review. So I think, i need to upgrade the brake at least to SRAM Avid BB7. But I'm not sure because the Avid BB5 are using 140mm rotor, can I use SRAM Avid BB7 using these rotor, or I should upgrade to 160mm rotor and do I need adapter to upgrade the brake. Need some input.
You can directly change for BB7 without needing new rotors. 

If you want 160mm rotors with BB7, you would need new caliper mount adapters. Changing to 160mm rotors will give you slightly more stoping power, but whether it's worth it, is completely subjective. 

It's pretty straight forward to do. Once you get the bike, you can ask a specific question about it.