Before committing to an upgrade on a car, I always look at the 0-60 acceleration time, 60-0 braking distance, top speed, and g-force numbers. I make it a habit of doing this, because just one upgrade can cost you a few feet in braking distance, or reduce your 0-60/0-100 and top speed figures. I’d also pay attention to the wheels you install on a car. It might be down to simple ignorance, but I recently discovered, that for whatever reason, heavier wheels actually can improve braking distance, which is just as important as acceleration figures are. In turn, I also found that lighter wheels can also hurt performance figures. The same can be said for various engine, suspension and gearbox parts. Also, I tend to stay away from installing oil cooler and intercooler upgrades, because they can add a great deal of weight, with little power gains in return. Rollcages are another thing that can really help a car overall, in terms of braking distance, but on the other hand, can yield no positive results at all.
I by no means, consider myself an expert tuner and used to be pretty poor at racing, and like yourself, went looking for any sort of remotely helpful guide, in hopes of improving my skills. I started taking tuning seriously in FM4… After reading countless guides, and just as many videos, I decided to go it on my own, and started experimenting with the tuning menu myself, since these guides were of no help to me. It’s probably obvious, but tuning is very much a trial and error ordeal… I’d start by messing with a few basic settings: roll-bar stiffness, camber, ride height, gearbox and differential, apply my settings, and see if that either hurt or improved my lap time, and then adjusted various things accordingly. I gradually began to improve my understanding of tuning in Forza 4 and 5, and consistently was able to lay down top 1% lap times in FM6 and Horizon 2 and 3 as well.
The thing about the guides and tutorials that people make, is that they’re making them from their particular driving style/technique and isn’t something that can universally be applied to every other person. Each person I’ve come across has their own way of doing things. Some, like to brake late, while others prefer to earlier. The same can be said about racing lines, one person might like to attack from the inside, while another might prefer an outside line.
Like I said, I don’t claim I’m an expert at this sort of thing… Just wanted to give my input, since I too was once very confused by tuning and upgrading.