Don’t mean to beat a dead horse here. But I agree with the above approach of playing with what you have, and adding incrementally as you see fit.
We started out with A&A Global 1940 as our base, along with the other 2-3 A&A games we had on hand. We did the same thing as above too, and just used chips to denote the different types of units. A tank with a blue chip was a light tank, white chip heavy tank, no chip medium tank, etc.
But over the last few years we’ve dipped in to some pieces. We personally like the aesthetic of a unique unit for each type, but also find it easier to remember that than a color wheel of chips! This is much more expensive of course, but doing so in pieces isn’t so bad. But everyone has different levels of disposable income. Dropping $100-$150 divided between the three of us from time to time hasn’t been so terrible. Using the HBG markers is an obvious great way to go. You can utilize your OOB infantry as Militia, Marines, Airborne, Regular Infantry that way, for only the cost of the markers.
If you do want unique HBG sculpts, I’d recommend starting by getting the base set pieces: Cavalry, Militia, Mountain Infantry, coastal subs (though I think the original game subs make great use for these though too), TBD’s. Get some white “Dutch” units for the minors/neutral powers too to flush those out if you want them.
From there, I’d expand the more unique units. Make the A&A OOB tanks your medium tanks (most commonly used), and buy some light tanks for each. Buy trucks for motorized infantry. Use the OOB Strategic Bomber as your heavy/strat bombers, then buy a few medium bombers. From there you could get marines/airborne as well.
After that, you could focus on fleshing out the rarely used units: heavy tanks, heavy strat bombers, jet fighters, advanced subs, etc.
My biggest hindrance now has been buying unique ship sculpts for V3. I’m having a hard time pulling the trigger on buy unique sculpts for light/heavy cruisers, battlecruisers/battleships/fast battleships. That’s been a big change for me that we’re still trying to decide what’s best!