I guess my plan of being a one car man isn’t going to work and I need different wheels for different races.
It depends what you want to do - each week new challenges go live and a good number of them require cars of a certain category (which could be something like Super GT, Retro Rally, Hypercar etc, or even specific manufacturer, or sometimes something a bit more random like 'heavy' or 'expensive') and also a certain class (the game is broken up into D, C, B, A, S1, S2 classes, and also the unrated X class which isn't used for races).
So for example, in this week 1 of season 1, there's a speed zone challenge (maintain a speed above a certain number through a couple of corners) that has to be completed in an A class BMW.
The weekly challenges are what keeps the game fresh after you've done everything on the map. Another thing to consider is that the game's 'accolade' reward system also has rewards for completing races across all the different categories and classes - Forza Horizon styles itself a massive car-based playground, and aims to have players switching around cars quite often as needed. It's not a pure racing game where you find the exact car & tune that works for your driving style and take on all-comers.
Re: tuning cars
You can apply tunes made by other people if you want to avoid slapping random parts into a car and making it undriveable. The game is too new to have any player rankings yet but over time the highly-ranked tuners will emerge as before.
If you do want to play around with upgrades yourself (because it is fun, but it can be a chore when you have to quickly build mulitple specific cars for events) then my advice is to slap on 3 things first - a driveline change to AWD, the highest weight reduction you can find, and the best brakes. This makes virtually every car launch faster and maintain far more speed through each corner. Tyres are the other important upgrade, but those depend on what type of races you're looking to do as there are specific ones for road, offroad etc. I always do these 4 things before upping the power output.
If you're trying to keep a car in a particular class and those upgrades take it over the limit, I recommend starting with a lower-rated car. I tend to find that a B class car upgraded into A class will be a better machine than a stock A class for example. Obviously if you're trying to squeeze slight gains out of a highly-rated S2 hypercar then there's not much wiggle room, but most car categories exist across multiple classes so there's incredible variety to be found.
