They always ask you those "why are you here?" questions.
This is very common question Schengen countries immigration can ask you upon your arrival "why are you here". You can simply answer you are here for that purpose (a justifiable reason for your travel) also you have connecting flight to your destination which is Germany.
I've heard that you need some money on you to be eligible to enter,
is this true?
Yes you have to have at least 50 to 60 Euros per day if you are planning to enter into Schengen zone plus your next or return ticket as just @pnuts answered. They usually don't ask about money, but on the safe side you should always be ready to provide at immigration if asked by an immigration officer. Also it doesn't have to be cash only, it can be cash, travelers cheques or credit card/debit card (preferably available credit in form of statements) which could convince the Immigration officer that you can maintain yourself in Schengen countries.
If so, will cash money in my home currency, USA dollars, be sufficient
or will I need to exchange it for Euros?
Proof of sufficient financial resources can be asked to support yourself during your stay.Cash money can be in any form of any currency. It can be USD, EUROS, GBP or any major international currency, but should be equivalent to at least match with your duration of stay.
Is there a minimum amount of money needed to enter?
If you have a passport which needs a Schengen visa, then you must have 50 to 60 Euros or equivalent for your entire duration of stay in Schengen countries.
You can carry up to 10,000 Euros. If arriving directly from or traveling to a country outside the EU: amounts exceeding EUR 10,000.- or more or the equivalent in another currency (incl. banker's draft and cheques of any kind) must be declared to customs.