The caffeine in coffee, if you drink it every day for how long? I don't know, but it can lead to headaches once you stop, so that day that you don't have coffee you will have a headache, and you will have a headache every day for about a week to 2 weeks. Due to that happening to me I now only drink coffee once every other to every 3rd day.
Headache medicines like Excedrin have caffeine it to help speed up the delivery of the meds. While you could treat a headache brought on by cutting coffee with Excedrin, all you are really doing is still taking caffeine, so you're not getting off the caffeine addiction by taking Excedrin.
As far as acid goes, yes, dark roast has less acid and less caffeine than light roast. However, the least amount of acid you can get, besides going to low acid roast, is cold brew, cold brew coffee made by steeping the coffee grounds for 12 or so hours, not by cooking the coffee then pouring it over ice method, the steeping method reduces acid by an estimated 60% over cooking.
The other interesting thing is, it is a well-known fact that caffeine can improve the performance of a cyclist, as well as other sports, BUT, if you drink caffeine (coffee) every day your body gets use to the caffeine intake and you won't see any performance improvement, the only way that works for performance is if you rarely drink coffee or any other caffeine product.