--Bitrates (I chose 115200 with Nano and Oudie, any reason not to?)

The higher the bit rate the more sensitive it is to noise and
interference.

Higher bit rates only help if the data being output needs the higher
rate - for example if it only outputs 4 x 100 bytes of position
information in a second then you only need 4*100*8 = 2400 bits + some
safety factor to transfer it all. Systems with flarms and high
frequency GPS systems will need higher bit rates than ones that just
have a standard GPS logger. Also, if you download the trace over this
interface you will want it to be as fast as possible.

--Internal GPS or GPS input from Nano?

I would always use the GPS source you would use for task scoring or
badge submission as the other one may differ quite a lot.

--It turns on with the master switch, any way to make it turn off with
the master switch too?

I guess the risk with this is that your trace would stop with a
battery failure, rather than using the nano's internal battery