I found a DC-DC buck (step-down) converter on amazon that will drop my 12V supply to a 5V 3A max with micro USB already attached. can anyone foresee any issues with using this following product in particular? Am i right in saying that although 3 amps are available, the pi will only draw what it needs?
https://www.amazon.co.uk/Supply-Converter-Module-Output-Adapter/dp/B00E5VH1OI
You should be fine, I run a similar setup. The main problem you may face is your power supply being unstable like fore when my battery voltage drops below 10 volts my Pi turns off also be sure that your supply is clean, you may want to place a capacitor between the device and the power supply. Finally don't forget that just because the device is capable of 3 amps that doesn't mean that your power supply is. And yes the Pi only draws the number of amps needed. Also that adapter has the incorrect USB plug.
First a quick note: The mini USB cable on that will not fit into the pi. The pi uses a micro USB and they are not the same shape. However, you could still use it, keep reading.
Am i right in saying that although 3 amps are available, the pi will only draw what it needs?
Yes, that is how electricity works. However, beware that stuff connected to the Pi can cause a draw exceeding the limits of the polyfuse on the microUSB supply jack, so unless you want to experiment with seeing how long that takes to reset (or what it can handle...), avoid that. I believe the limit is 2 A on all models but the 3, which is 2.5 A.
Also beware if you backpower via normal USB or one of the 5V pins this polyfuse does not provide any protection. Presuming this 2-3 A limit is there for good reason, in that case you want to make absolutely sure no more than that will be used.
If I were doing this I'd chop a microUSB cable and wire it to the supply so you do not have to backpower. This also ensures the power goes through the onboard regulator first (which it certainly won't do via a 5V pin).
It may also be possible to get a mini->micro USB adapter.
