Buying and reading a couple of books would probably be a good idea if you're looking to purchase one of the most widely faked military weapons in America.
Here are a couple of titles:
'The Krag Rifle Story' - Mallory and Olson
'The Krag Rifle' - Brophy
'The American Krag Rifle and Carbine' - Poyer
The first two are literally 'bibles' - the third is merely 'available' - but may be your only reference for awhile.
If you've been looking at the various gun selling sites online - you already have a good idea that you're going to have to part with some serious money if you really want a nice, un-screwed-with original configuration weapon.
The Model 1899 will be the most common - often being a rebuilt piece from the post-Spanish-American War/Philippine Insurrection era - but in order for it to fit into the whole 'Rough Rider' Impression, it'll have to be an earlier Model with a sling bar and ring.
My GG-Uncle was issued a Model 1896 when he rode with G Troop.
Hard to find in good condition - many were sporterized and/or used to death as deer guns.
Vaya,
Scouts Out!