Never buy blueprints, every next step of blueprint is more expencive than first one. Play games with "supplies" option. If you see in game a supply box, conquer this province and you will get a blueprint or ressources. If you play parallel rounds with supplies you can gain several blueprints everyday.

use gold for spying and espionage. If you have 10,000 gold, you can see all enemy units. It is VERY useful.

Yeah, definitely dont buy blueprints. I've been playing for a long time, and just unlocked all of the blueprints over time. It actually isn't too difficult.

I have played very few maps over the last 5 years because I like to play the maps I play with full attention, I still do not have all the blueprints available but it never held me back. Also, for the start of a map you do not need blueprints because the elite units can only be built after quite some time. Most maps I played finished before the elite units could even be built.

In 1.0 elite units could be a game changer in ranged units because their range increased. In 1.5, not so much.
No real need to buy them.

You probably do not need to buy any. However, if you have loads of gold and you want to then really I would purchase what ever units you use the most of. No one can really say what the 'best' one is especially in 1.5 (no extra range for arty) as everyone uses their own strategies.

So it really is simply about what troops do you use the most? And then purchase the blueprint for that troop

If you want at least one blueprinted unit a round for sure: milita is researcheble as first blueprinted unit, especially for panasia or comintern doctrin it is "should have" unit, especially on historical map.

blueprints are not important in early days of games as you cant research top levels yet