I have recently been going over the units selection with three different faction (Averni, Rome, Getai) in order to find a proper units, and I have had a better success in mixing a good units with a medium ones, so that I will forfeit mine previous selection of elite-units with its estimated minai from 45000-50000. As an answer to your question which are ‚good units‘ then I would say that is units which are over 2000 minai and the infantry have a defense of 22 and up, while the medium units are from 1500-2000 with a defense of 18 to 22, but the cheap units are under 1500 minai and with a lesser stats.

However even that I have a good mix of units then it still feels about 3000 minai short from having a full stack army (need to two slots of units). Here I will make an example:

Mixed units of Averni.

Belgae Heavy cavalry 1
Gallic noble cavalry 1
Celto-Germanic cavalry 1
Ligurian cavalry 1

Hellenic Slingers 4 (upgraded 1)

Gallic Noble infantry 1
Gallic naked fanatic infantry 1
Averni Nobles 1
Gallic heavy swordsmen 1
Helvetii Phalanx 1
Belgae swordsmen 1
Boi Swordsmen 1
Norica spearmen 1
Rhaetic axemen 1
Druids 1

So it is 4 cavalry, 4 ranged units, and 10 infantry
If I would have about 3000 minai more then I could add let‘s say 1 North Gallic swordsmen and 1 Naked celtic spearmen.

Here is another example if I would have a picked Averni army:

Gallic noble cavalry 2
Celto-Germanic cavalry 1
Ligurian cavalry 1

Hellenic slingers 4 (upgraded 1)

4 Averni Nobles
6 Gallic heavy infantry

So it is 4 cavalry, 4 ranged units, and 10 infantry
If I would have about 3000 minai more then I could add let‘s say 2 Boi swordsmen.

The similar thing has applied when I attempt to create a mixed army with Getai and Sweboz. I have noticed that some native units are more expensive then other general units even that they have a lesser stats. For an example then a Boi swordsmen are cheaper and have a better stats then a Belgae swordsmen, and the similar thing applies about a classic hoplites in comparison to other spear-units with Getai. Does it then not encourage players to have a lot of such units in comparison to their native units? Would it not be historically incorrect to field many such units to armies while neglecting the more native ones?

But if the money limit rules (36000) are mostly made to preclude the players from having too many elite units, then could then not be made rules which limit elite units to only 4 infantry and 2 cavalry and 2 ranged units (or maybe that only one elite infantry can be picked from each sort in regard to medium sized factions – excluding AS, Rome, Carthage), while a little increased money (3000-4000 minai) are supposed to encourage a more flexible selection of native medium units that belong to each faction.