Alright so I’d recommend using Blender for this purpose. While it can be hard to learn, with a little persistence it’s incredibly powerful and versatile. I’ve been a 3D Modeller on Roblox specialising in vehicles for around two years now, and I’ve never looked back from using it.
Making the cars themselves in this ‘low poly’ style is relatively simple and you should be able to make your first one within a few days of picking up the software. If you make sure that each part you want to be colourable, or a different colour from the rest, is a separate mesh within the finished Roblox model, you will be able to change each part’s appearance within the game as if they were individual parts. Some major advantages to using these meshes over part built ones is that the mesh can be far more optimised, and you as the modeller have far greater control over the small details with the greater variety of tools available in dedicated 3D modelling software
Judging from how you say you’re planning on making a game, you want the cars to move etc. and not just be static display items. An important thing to note is that mesh collisions are ‘optimised’ by Roblox when you import the meshes, so it can be helpful to make low-detail part colliders to avoid janky collisions, especially if you end up making the car meshes more detailed than you originally intended.
In terms of learning the software, your best bet is to learn the basics from an online tutorial and then start learning techniques specific to modelling cars. There are loads of useful resources out there to help you learn: YouTube tutorials; forums and guides - and you should use all of them; as well as just jumping in and trying stuff out to see if you can do it. Although, be mindful as to what version of blender they are referring to, as many use Blender 2.7.x rather than the newer 2.8, which has a completely different UI and controls.
tl;dr - Use blender, it’s way more powerful than studio for this purpose and is worth learning. Use whatever you find online (doesn’t always have to be car specific) to learn and you’ll be making cars in no time.