Change BB and chainring/crank help.
 

My bike came with an FSA Omega chainring/crank and Omega BB. The rest of the components are Shimano 105. I would like to replace the BB and chainring with 105 components. What measurements/specs do I need to have to insure the proper fit to the bikes existing dimensions? I realize that the 105 group will not have the same 46/30 chainring sizes as the FSA so that is a given. Just wanted to see if what I want to do is even feasible and cost effective.
Thanks in advance, Frank.

You will need a 105 Hollowtech crankset(or any other Shimano Hollowtech road crankset) and the SM-BBR60 bottom bracket.

Would be helpful to know what bike you have. The 46/30 chainring option is often seen on gravel bikes.
this: https://www.jensonusa.com/Shimano-GR...Speed-Crankset
is designed to provide a bit of extra chain clearance for the wider tires on gravel bikes. A standard 105 crank would not have the extra clearance or the 46/30 chainring option

A couple of possible lower cost options, includes BB. As stated above, what bike this is for would be helpful.

https://www.ebay.com/itm/NEW-Shimano...97.m4902.l9144
https://www.ebay.com/itm/Shimano-RS-...97.m4902.l9144

I am looking to change my Topstons alloy with 105, except for the chainset. The clearance issue is something that can't be adjusted for I am guessing and the lack of 46/30. Thank you for your responses. Much appreciated.
Frank.