The equipment selection would need to be playable in the MA. I believe you could set it as a date. Like all equipment that was introduced through a certain date. December 1942 or January 1943 seems a good cutoff because they would have access to Spit9s, 109-G2s, Zekes, P-40s, B-17s, Sherman’s, T-34s (not the 85) and more. Variety would be a positive. I would eliminate the free to play players from having any access to the currently perked equipment whatsoever except in special arenas like FSO, scenarios, and Monday night madness. There has to be a reason for them to step up to the $15. Limiting perks to a set value which resets every month also encourages that step up to a subscription by giving them a little taste of what they’re missing. I would start low with access to post 1943 rides requiring perks for free players. You could always open up more perks for them if you’re feeling generous. Or you could have sales on perks.
Well, the B-17G entered service in June of 43, and the T-34/76 in February of 43. I think that general timeframe would be fine, though there would probably be folks perfectly happy to fly Spit 9s or 109G2s and never pay a dime. Those planes are perfectly MA viable in my opinion. Not as fast or capable as late war stuff, but also have to keep in mind this would theoretically bring in a larger pool of people with those same planesets, so there would be more of them out there too, not just all P-51s, Yak 3s, and D9s like we have today.
My opinion of course, but I feel like the free selection needs to be fairly limited and needs to ride a fine line of not terrible but also not good. Basically, barely MA viable. You want people to have success and enjoy but also want to entice them to pony up and access all of the planes to enjoy the game even more. I would consider the likes of the Spit 1, Hurri 1, P-40C, C.202 as not feasible to play in the MA, but one step up like the Spit V, 109F, P-40E, Yak-7b can provide some opportunity. Of course, no chosen planeset is going to make everybody happy.

To equate it to something like War Thunder or World of Tanks/Ships and being uptiered/downtiered, not everything has to be equal, just needs to be in the realm of possibility.
If it were up to me, and it isn't, I would have the following as a rough 'free' list:
Land Fighters:
P-40C, P-40E (maybe), P-39D (maybe), Hurri 1, Hurri IID, Spit 1, Spit V, I-16, Yak-7b, Ki 43, Bf 109E, Bf 109F, Bf 110C, C.202
Land Bombers/Attackers:
A-20G, B-25C, C-47, He 111, Ju 87D, G4M1
CV Planes:
F4F, Sea Hurricane, A6M2, SBD, D3A, B5N, TBM (maybe)
GVs:
M8, M4 75, T-34/76, Panzer IV F, SdKfz 251, M3 (maybe)
Something like this gives a decent variety with some capable planes for all aspects of the game, while saving the better/more efficient aircraft for the paid or perked tier.