Everything I've seen/read so far is that the new game is a whole new piece of software. That isn't to say you might not be able to buy it digitally on PC. The base and characters from the first Infinity will work with the new game, so maybe there's an option if you're on a PC to simply download the new one. I can only speak to consoles though, and for that it's definitely a whole separate physical purchase. While the old base and characters will work with the new game, the new characters won't work in the old game.
As far as buying the original one now, I would advise against it, unless you have a particular interest in one of the game sets. The toybox mode in the original version keeps most of the more interesting blocks behind a lotto system, where you earn a chance at one of fifteen new toy packs every time your character levels up. There's never a situation where you don't get a new pack when you level up, but there are like five different towns-person packs and four racetrack packs, and three castle part packs, etc. so if you really want to go in and make nothing but F1 tracks, you're still likely to end up with a lot of stuff you don't care about.
FWIW, this is one of the changes to the toy box for the version 2; you'll still need to acquire some of the packs, but instead of being randomized, it'll be things you can more specifically pick and choose from.