Be careful with old second hand units as the air receiver could have rusted from the inside and be rather dangerous.
And as said above look for the highest CFM you can afford on a reasonable receiver. Make sure it is FAD (free air delivered) and not displacement, as the FAD is what you actually have to use.
As for tools, the easiest way to judge how much air you will need is by how the tool functions and how you will typically use it.
For instance a tyre inflator is simply putting air into an enclosed space at a much lower pressure than the stored volume in you receiver – so it does not take much air.
An impact wrench or drill have a vane powered rotor in that spins to provide the force. The air is then exhausted so it will take a large amount of air if you sit with your finger on the trigger. However in practice you drill or screw/unscrew over a short time period and then put the tool down, so the net air consumption is relatively low.
An air vacuum invariably uses air pressure passing over a machined venturi to create the vacuum. This is a real high user of air and whilst beneficial to industry for the average diy’er I’d advise against their use.
Hope this helps a bit, and I could go a lot deeper into it if needed 🙂