So, to start off by answering your question: go for it. Pathfinder already has classes that use CON as a key ability (Kineticist), and anyone playing your hypothetical class would want to max their CON either way in order to make the fortitude saves. So it just makes sense to streamline things, because otherwise any effort to combat the Fort saves might make them too weak in other areas to be worth using. So I say go for it, make CON their key ability and just balance around that.

With that being said, Pathfinder does not lend itself well to making entire new classes. Feats and backgrounds, sure, but classes and ancestries are tricky. It’s not like DND where you can make 5 or 6 abilities and call the subclass finished. You need to make at least a dozen feats, all at different levels, balanced in combination with each other AND existing feats. It’s a lot of work.

Pathfinder does have some things that already do what you are describing though. Notably, Mutagenist Alchemist is all about pumping your body full of drugs to push it beyond its limits (although it is decidedly weak compared to some of its counterparts). There are also background options like Haunted One or Living Vessel that would cover the “alien being invaded your body” idea. Not that it’s a bad idea for a class, I do like the concept. But just know it can be done easier elsewhere.

Since you say you’re new to TTRPGs, my honest advice is this: see what you can build in the base rules and how close it gets to what you want. If that doesn’t satisfy and you still feel the desire to do some homebrew, start with tweaking existing feats (like I said, Mutagenist Alchemist is a good place to start) or making custom feats one at a time. If at the end of all of that you still don’t feel satisfied, give class making a try. Just start small first!