I've toyed with ideas about buying hits but haven't implemented or tested anything. The concept is somewhat awkward in Anarchy where, in theory, every action is an opposed roll. (For example, Very Easy actions "oppose" you with 4 dice.) Now, in practice, I don't like the extra dice rolling so I often default to thresholds similar to what SR5 uses. In that case, buying hits is a good way to guarantee that you get the 1 or 2 hits that you need, which cuts down on dice rolling and speeds things up.
I would use the same conversion rate for all weapons. To echo Gingivitis' point, don't make rifles more attractive than they already are. You might be able to make an exception for machine guns, since narratively they're so much different than small arms, even automatic small arms.
Gingivitis' suggestion around Cooldown is an interesting idea. I would use it as-written so you don't have to support the bookkeeping of "how many times in a row was this used?" Anarchy got rid of ammo for a reason; similarly, I wouldn't want to track how many times in a row someone has done something. Keeping it to a binary response ("did they do the same thing last time, Y/N?") is about as much as I want to do personally.
As for the AP approach, I might not be in the best position to opine on that because armor is one of my least-favorite aspects of Anarchy. I've been experimenting with different house rules to bypass the "armor as hit points" approach that Anarchy uses, so my experiences won't be directly applicable to your AP suggestion. I will say that, from a mathematical standpoint, losing 4 dice to get 1 point of Armor Avoidance seems like a bad deal. I'd be more inclined to make it a 1:1 ratio, so that every die you give up gives you 1 point of AA. This not only bypasses armor (which, as noted, is a plus in my book) and turns into a fun risk/reward equation for the player as they try to judge how deeply they can cut their dice pool before they face a substantial risk of missing the shot altogether.
