Hello Heniz,
I'm the founder of a British WW.1 reenactment group here in California: 2nd Battalion Lancashire Fusiliers. To add a little to what 'Oskar_2ndChev' has informed you. All three periods, American Civil War, WW.1 and WW.2 are about on an equal par as regards to cost to the individual. However, that should never deter anyone from getting in and enjoying the period with like minded individuals.
I appreciate your interest and want to portray your Grandfather's unit, but there is quite an overflow of German units at present, and the Allies from both World Wars are being left in the dark. Pretty soon, the German units will have to fight amongst themselves, or wait in line in order to engage an Allied unit or two.
In reference to your question about the American side of things for WW.1. The correct rifles to use would be either the '03' Springfield, or the Model 1917 Enfield. The Pattern 1914 Enfield in .303. cal was not issued to American troops, unless they were directly attached to a British unit during the latter part of 1917. However, any American unit that you portray will only have one particular type of rifle in use; either the Springfield or the Enfield... not both. Incidentally, Sgt Yorks battalion used the Model 1917.
The German rifles of the period are out there in their original military spec, but if found are commanding ridiculas prices. Many are still found in europe, but you'll need to find an FFL dealer who is willing to carry out the paperwork for you.... and this comes at a price! The ongoing craze (eating up both Allied and German rifles) of sportizing, is where you'll find the item you require, but you'll need to find a good source of original parts to bring it back to military spec. Visit as many 'Gun Shows' as you possibly can... you might get lucky!
Hope the above has helped?
Seph