Okay so here's the really general idea.
I have 4 tanks (1L each) of which i run tubing (no more than a meter long) from each and then, using a series of electronically controlled valves, these 4 tubes funnel into one which is passed through a fluid pump and then a flow rate meter and into an "output" tank. The idea is to be able to select one of the tanks by controlling which valves are on and off, and then turn on the pump and move specific volume of fluid from the selected tank into the output tank. Ive designed this in this way to make the project more affordable. I understand there may be an issue with my design and my use of the single fluid pump as there may be situations where i require the pump to run "dry" for a few moments until the fluid reaches the pump, but i would like to know if this is a something that can be tolerated or if this design simply isnt feasible?
If it is not feasible then i would greatly appreciate any workaround ideas and you should probs just stop reading now cos the rest will be pointless aha.
If it is feasible then please continue reading as i have a few questions:
I wish to power everything from a single power supply, this includes the arduino, sensors and valves. Are there any cheap options out there for 70W+ power supplies? Also, obviously i will need to step the voltage down at different points and i would like to know if buck converters are a "safe" approach for this?
Also how do i go about switching my pump. Do i need a transistor to drive a relay to switch the pump or is it possible to switch the full 6A to run the pump with just a transistor or is it possible to switch the relay without a transistor? I have searched the web tirelessly and have found a combination of these being suggested and im just not sure which is best.
I am fairly new to all of this and would greatly appreciate any guidance 
(If you need more info or for me to elaborate on anything just let me know)