Donīt buy planes as a young airline. You will need the money for many other things, as your revenues wonīt be good until you have a good reputation (CI and RI).
You can narrow your search down on the new and used plane pages by filters.
Any plane is a good one as long you use it wisely and obey some major rules. Obey fleet commonality, fly as much planes of one family as possible before acquiring a new family to keep costs down. When chosing your type you should make sure there are enough of them on the market, and if individual planes are not too old and out of production, to be able to acquire new or replace old ones.Find routes for your planes, not another plane type for every route.
Find out, what kind of routes are the most un(der)served from your airport (SH,MH,LH) and which airplane can serve most of them with a good LF at least once daily.