So how difficult is it to get all trophies?
It's gonna take some time. There's a trophies for leveling up online. Won't be hard, just time consuming. You get like....half? Or like a trophy or two over half for just beating the single player, which isn't that hard. But it's just the online ones, like reaching X level online.
The hardest trophy to get will be the one for winning a bet on a 2x streak, but even that is not that difficult. After a race just view the results of that race and bet against the players that are consistently finishing in the bottom of the pack. If you're at least decent at the game shouldn't have any problems beating them enough times to get the trophy. Most of the other trophies you can get by just playing the game naturally.
"When life gives you lemons don't make lemonade, MAKE LIFE TAKE THOSE LEMONS BACK!!" - Cave Johnson, Portal 2