With all the characters and other customization items that have been released since the game launched, it's definitely gonna take a while to get the coins. That said, around the time I finished the challenges for each GP since like GP2 or 3, I tended to have made back all the coins I spent on the new pit stop content. And I'm not actively farming, or optimally trying to get coins at all, just idly playing really. And of course Beenox has, since the launch of the game, added more options to get coins easier: weekend bonus during offline play, gold wumpa, adjusted coin rewards so each track is roughly the same payout, additional modes that reward you with decent cash like Ring Rally, and of course once GPs are over, what used to be GP challenges will be retained to earn coins.
Obviously if you want to have everything right this moment, it'll be a grind, but if you're content unlocking the stuff you really want first, and then just occasionally buy some shit off the store, you're gonna be fine.
At least all of the characters, skins, kart pieces etc. do not change the gameplay at all (outside of kart size I guess?), unlike in say MK8, where each kart piece changes your stats.