If you haven't shot much position shooting before you'd do just fine with surplus M80 ball (i.e. if you aren't very familiar with shooting with a sling). You might buy a box of match stuff for the long line if you're going to shoot the full distance to make life a little easier with the wind. But if you're not shooting 300yds or more, you won't need to compensate for wind.
I'd only use the 175's at 600yds. The rapids are tough enough with the M1A without adding even more recoil into the equation. The 175 has got a slight advantage against the wind over the 168 on the 600yd line, but not a heck of a lot (for a 1mph change, I still use one half minute click for either bullet). If I was buying ammo, I'd shoot the 175 on the long line and 168's everywhere else. Black Hills or S&B will probably be cheapest if you don't reload.
I'm currently shooting the Speer 125TNT bullet (half the recoil of the 168) at 200yds and the 168SMK at 300 and 600yds (I ran out of 175s and I have 168's coming out of my ears). I got a Kreiger 1:12 after I shot out my other barrel.
Ty