Best fleet for start

Should I invest in fleet combined of ATR and A220 or of Q400 and A31x?

Solely depends on your market. It wouldn’t work in Nepal. Neither with a transatlantic company. It all comes down to which market you serve. And what your strategy is.

Ill start in mexico. but im thinking about getting a a319 to fly transatlantic

ATR vs Q400 is entirely a matter of taste and whether you need the smaller (ATR 42) and cargo versions of the ATR or the higher speed and slightly lower operating costs of the Q400.

Don’t start longhaul straight away.

atr definitely works. mexico is quite big, so you‘ll need to cover relatively long distances on domestic routes.

E-Jets will work well, also for most of the routes to the states.

a320 family will also produce nice revenue and includes some long haul options without adding more maintenance categories.

cheers,
dante

for now i decided to operate Fokker 100 and actually im considering AN-148 family

Mexico is easily big enough for all jet fleet. As a beginner go with E-jets, CRJ or A220s. I would have gone directly to A320N family just to get that low cost per seat, but you need a model which can fill them.