Anytime you build a "first" rifle for competition, the wise choice is to go with what is common; with what has worked for other people. When everything is new to you, it is always best to cut down the number of potential problems.
So, what Al said. Rather than pick a barrel manufacturer by the number of groove they use, pick a manufacturer that other experienced competitors use -- Bartlien, Broughton, Hart, Kreiger, Shilen, to name a few. This is not to say that other barrels are not potential winners -- for example, one of my best competition barrels was a Pac-Nor, and I'm going to try a couple Douglas barrels this year. But these will go on rifles that are already proven over several years of competiton.
As to the forcing cone angle, again, what Al said. 1.5 degrees half-cone angle is well proven. There have been been both arguments and test that other angles might offer some advantages, but the results are all what the doctors call anecdotal. No statistically significant proof.
The time to experiment is after you have some experience under your belt. The questions you are asking concern barrels, and they are a disposable item. When you have a rifle capable of competing at a high level, your experiments -- changes from what is customary -- will be far easier to conduct.
FWIW
