Good test. Should we increase gun & maintain reliable first or increase speed and agility first, for the same amount of xp?
This is where it might get complicated.
Some time ago I did a test where I nailed down an upgrade procedure that involved like going +1 engine, then +1 guns, then back to engines for 2 steps, then back to guns, and so on. It was complicated, took a lot of time to test, and ultimately didn't help me play the game better because I didn't factor in range upgrades.
So, I'll give you the easy to remember version.
First of all, bank 95% of air XP prior to researching 1940 fighter only spending it on things like the research boost air force spirit and spending any overflow air XP that would get lost due to the 500 XP cap. When research on 1940 fighter is complete, go +5 engines, then add guns and range as needed (I can't tell you what range you'll need in your game, that's on you). In SP, as long as you know you will be facing significant air combat, reliability of 50% is a reasonable target. Lower is sustainable, but once you clear the skies, you might lose more planes than you want this way (if it even matters by then).
You will be exercising wings using older aircraft, off course. Forming new wings with older aircraft, they exercise on the map as new planes filter in and when they get capped on XP, they should be good to go. Older planes tend to have higher reliability, so you won't lose lots of 50% reliability planes to training accidents.
I emphasize engines first in this scenario because it's the no-brainer choice. No matter what you intend to do with a fighter, engine upgrades are useful. Guns and range are more situational, as there might be times where the range is more important or the guns are more important.
Note that heavy fighters are not the same, so if you have heavy fighter specific questions, that's a whole other thing in terms of design philosophy.