If your Launcher.dat is not the 3.2.1 version, you will need to replace it. As long as you have the 3.2.1 version of the Launcher, you can use System Settings > Update System Software to perform the update. Just make sure you're in your emuNand when you do so. Since you verified you have an Old 3DS, you will be able to run the latest 9.7 in emuNand.It's been a little while since I updated Emunand with my gateway, if I remember correctly all I have to do is replace the launcher.dat, boot into gateway mode and then go to system update right?
If you make the backup from your own retail card, then there is nothing you need to do. If you obtain a backup through other means, you will need to add a private header to the backup in order to safely play online. There are multiple guides on how to do this, and the steps are a bit different based on what equipment you have.I've had trouble finding the answer to this pardon my bad searching ability. But if I were to load a backup of a game that can play online would I be able to play online just the same as my original copy of the game or do I have to do another step. Also if I wanted to buy a 3ds from my local best buy that is compatible with the currents flashcarts which models would I be looking at?(I live in Canada)
As for obtaining a compatible 3DS, there's a bit of luck involved. There is a thread that tries to link serial numbers to starting System Version, since serial number and manufacture date should be somewhat linked. You might be able to return it if it turns out to not be compatible, I'm not familiar with Best Buy's return policy. That's something I'd look into before purchasing, though.