Hi hunypuf, and welcome to Salehoo :)
What's in a name?
To be honest terms like agent or distributor are quite generic in a modern market, even the term wholesaler is used in some cases where the supplier isn't a true wholesaler at all. They're terms, while used to have a specific meaning, are now nothing more then marketing terms in a lot of cases.
On the face of it, I would assume an agent to be a "brokerage", one that doesn't supply products themselves, but can put together deals on products for you from their sources.
I would assume a distributor to be a large scale supplier, one that supplies smaller wholesalers, not one that supplies retail.
As I said, they would be my definitions of the two terms, but as these terms are now used so loosely these days, you would really need to confirm what it is these people actually do!
Q2, if you are dealing with a broker (agent), then there will of course be a brokerage fee for delivering you their service, however that doesn't always mean that you will pay more than if you dealt with a wholesaler yourself.
Brokers have different deals with suppliers, and some might be able to source your products at or even below what you can, even dealing with a wholesaler direct. This is a question of research really, you would need to put the products and prices that a broker offers you up against those you can source wholesale yourself.
Q3, it's never too early to establish supplier sources, the stage of which your site is in development should play no role in making product sourcing decisions.