We Weberheads generally measure charcoal by the Weber Chimney. It may be a full chimney, a half chimney, or whatever. It is a standard unit of measure.
So, get a Weber chimney. You can put it on top for your propane starter, or just use it to measure. They are about fifteen bucks, so not a big investment.
When I am looking for serious BTUs for something like a thick steak, I always use a full chimney. For something like chicken, a half to two-thirds of a chimney is plenty.
On any kettle, it is a good idea to make one half hot, and one half not so hot, which means you are putting your coals on one side of the kettle. Using a chimney to start your fire helps a bunch for that. Having those two zones lets you move meats back and forth, so you get a complete cook without burning your meats. It gives you a whole lot of control over your cook.
To smoke on a kettle, you need to learn the Minion Method (search), because you will be mixing a few lit coals with a bunch of unlit charcoal, to get a good low-and-slow temperature.
Once you gat the hang of it, a Weber Kettle is amazingly versatile.