Supplying a voltage will draw current, you have to know how much or you might not get the voltage you desire it's something and you need to know because it will effect the choice or transistors you use.
Yes, the transistors are just to pass the higher voltage, 555s are low voltage devices only, I think the highest voltage you can run a typical 555 timer at is 15 volts, but they can supply enough current to drive the base of high voltage transistors. The choice of transistors depends on the current that this 300 volts will drawing. Could you describe how this is supposed to work a little better? Are you basically just charging up a plate that your experiment is sitting on? If that's the case you're right it won't be using much current at all