"2x4s, of course, aren't 2" by 4". They're approximately 1,5 by 3,5 inches."
Depends on who you're buying from. My Father purchases his lumber from an Amish lumber yard. They cut all their own lumber and anything you buy from them will be the size it is labeled as, if not slightly larger, and frequently cheaper than you can get elsewhere. My Father loves it and goes at least 40 minutes out of his way to do business with them.
As for using whatever measurements are offered, screw that. When I was overseas everything was listed in kilometers and I just did the simple math in my head for miles. The measures on all kinds of food products here in the USA have been listed in metric as well as imperial for a long time, and I don't know anyone that has started using the metric measures.
One of the biggest reasons I keep hearing for using metric is that it's easier. Imperial isn't really that challenging though. I could save a lot more effort by using velcro or slip on shoes, but I don't because it's a trivial amount of work to tie my shoes in the morning. Metric is like the Newspeak of measurement systems. I'll stick to using imperial wherever I please along with whatever anachronastic measures I want like rods, furlongs, and libraries of congress.