GBA FEs

You dont buy physical copies, pirate them, youre not supporting nintendo by buying used games, the only thing your doing is putting money into other peoples pockets.

FE7 all the way. Also, I agree with Nilla. just play a rom of it.

You don't happen to have a Wii U, right? They're on the Estore there for a low price.
If that fails, you could try emulators.

As for which game, I'd say FE7 has a more expansive prolouge that tells you all you need to know (it is a tad too long for my taste), but has a more challinging game where you can't grind all that well.
FE8 is a bit more beginner friendly. It's a bit easier and you can grind as much or as little as you want.

Both are top notch FE games though, in gameplay, characters and story.

The cheapest is FE6 because you cant find versions that arent romhacks

But its one of the hardest in the series so...

I would say emulator and if you want to pay for them, then just buy orbs because for how expensive they are, they should come with a game anyway.

Or go ebay I guess:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=fire+emblem&_sacat=0

I'm pretty sure that somewhere in the manual or on the case, it'll say 'not for resale' though. So you're really breaking the law either way....