The only current option I have to "build" a new building - is a Bank.

However, I already have a Bank.

Will I ever need a 2nd Bank?
Will my current one not be able to hold all my money? - so long of course as I keep upgrading it... ?

Firstly there is currently only 20 levels for each building, so no you cant just keep upgrading. Secondly the upgrades get very expensive and time consuming especially compared to earned revenue at the lower levels so it is easier and quicker to develop by building additional banks. I am level 11 and have 4 banks none of witch are level 20. However if you consider maxing one bank at a time to be the optimum strategy that decision is yours I am in no way an expert

Ah. Right.

I "get" that - thanks