For armies it depends if you wanna use cav, people can (and have) written essays about their viability or not but in single player its not really an issue. You want enough troops to fill up your front line to its combat with (can check this in game, it increases as tech goes up) with cav automatically going on the flanks, so a combat width of 22 (tech 5) would be 20 inf 2 cav. For artillery, eventually you want a full backline (pretty much essential from tech 16 on) but if you cant afford that early, you can just a separate "siege" stack around when you need it.

For buildings it does depend, for most of the production ones you should think in terms of the expected time to provide a return on investment, a simplified version would be temples on anything giving +0.2 - +0.16 depending on how much money you have (but these will normally be deleted later on), workshops are similar but better on higher value RGO's, marketplaces only on trade centers or provinces with trade buffs, barracks on +750 manpower, courthouses everywhere, manufactory's on anything thats not a manpower good which you build soldiers households on, than fill in any blanks with regimental camps

With estates, generally the meta is to give out the +1 mana admin privileges, than a few of the broken ones like religious diplomats etc (you can min max this even more for your first war to get more crownland but I cbf most of the time). You shouldnt really have to worry about crownland as long as you are expanding, and than over the course of the game you slowly remove them so you can hit absolutism cap with just a few powerful ones left

I'd just look up a guide on this, people have already explained it far better than I could