-The Full buckhorn or semi-buckhorn are obsolete and would look out of place on a bolt action. Also, they will cover a lot of your target. The flat top is the only way to go.
-You will enjoy shooting cast bullets more if your rear sight has plenty of elevation and some windage adjustment.
-If you don't have a front sight, buy your rear sight first, put it in its lowest position, and fire the rifle with a temporary front sight. You can make one with tape and cardboard and cut it off until you get the proper elevation ( slightly below point of aim at 50 yards ), then measure it to find the front sight height. Most bolt action rifles will require a tall, ramp front sight.
You should give some thought to a good receiver peep sight. The Williams FP has models that fit just about every type of bolt action rifle. Some rifles may be already drilled and tapped for it. The williams sight will give you all of the elevation and windage that you will need. The FP 'TK' model has target knobs that allow the sight to be adjusted without tools.