How do I find out what BB I will need for a bike?

Looking at a Motobecane Nemesis for a training/foul weather bike and want to convert the gearing from 53/39-11/23 to 50/34-11/34.

Has Shimano 105 RD and I assume FD, but does not have a Shimano crankset.

Trying to figure out how much it will cost me to do the gearing swap, and if I will need a new RD.

Thanks.

What crankset does it have ?? Should have a name on it, and with luck, they make a compact version that can use the existing BB.

You will need a long cage rear derailleur and a new chain, in addition to the crankset, and possibly BB.

Decide which crankset you will be using and get a BB to match its spindle length requirements. You can measure the one you have and use it if it is suitable but budget for a new one. You can often buy a crankset/BB combo at a good package price. Trying to find a crankset to match your existing BB is letting the tail wag the dog.

Many square taper cranksets are mostly interchangeable, as long as you stick with a double, and don't move to a triple crankset. There are a few exceptions, mainly with vintage bikes.