Yes, quite historically important. About 2 million made by three different manufacturers: Remington, Winchester, and Eddystone. Most were rebuilt by the U.S. government armories after WWI by taking various parts from the three different manufacturers and making "mix-master" rifles.
Mine is a Remington receiver with a variety of other manufacturer's parts. It has a Winchester bolt, Eddystone barrel, Eddystone sight, Eddystone bolt retainer, and a variety of other parts and accesories. You can tell by the fact that each part (for the most part) has either a 'W', an 'E', or an 'R' stamped on it.
They shoot very nicely. Biggest issue is getting one with a pitted bore or not. Mine is pitted, but shoots pretty well (I will find out more once I've taken it to the range for a thorough test...I've had this rifle since I was 15, or around 1984 or so back when you could get them for $150; thanks Dad!) I would not worry about headspace so much as most of these rifles would have been rebuilt at the U.S. armories and thus done right unless it has been sporterized (read, butchered.) What most M1917 'collectors' look for is parts-matching by manufacturer, pitting in the bore, and I seem to recall an issue with certain barrels which were incorrectly fitted by some other outfit that resulted in cracked recievers. Johnson barrels comes to mind.
They are big, heavy, and robust. With the bayonet attached they are 64" (5'4") long rifles. Some people don't like their cock-on-close action, or their dog-leg bolt, or their weight, or their length, or their looks, or their balance, or their smell, or the way they induce seizures when you fire them..
..okay that's going a little far but you get the idea. The point is that the M1917 rifle is an important bit of US military history. If you get your hands on one chances are that it, or at least some of its parts, were carried and possibly fired in battle or at least in training for our troops in the last century.
Try going to one of the many M1917 forums, perhaps
these folks as a start for your information search. They and other info you find might prove some of my points incorrect (I'm going from memory on this) but for the most part the M1917 is a pretty good rifle for all of its warts. It was good enough for the U.S. Army in the first quarter of the last century, and that is saying something.