Fragrances can last decades if stored properly, so I wouldn't be too concerned about that. As for how big of a collection to build or how much to spend, that's something only you can decide as you go. If you're just looking for a handful of your favorites, then I'd recommend making sure they are definitely your absolute favorites and would spend accordingly (there's always deals to be found on anything). If it's also about "collecting", then I'd see it as any other type of collection. Any hobby or collection can get expensive..it's all about how into it you are and how far you take it. It wouldn't say fragrance collecting is any more expensive than the majority of others.
Personally, I focus much more on adding fragrances that I absolutely love and know I'll wear at least semi-often. If I find there are some that no longer fit that, I'll look to trade them off or sell them. I love testing/sampling new fragrances more than I do actually collecting bottles, so I'll spend more time on that part of it and only purchase full bottles of my absolute favorites. In the long run, I likely see myself narrowing down to maybe a dozen or so, but I don't really have a set number or limit I have to abide by. If I like it and will wear it, I'll eventually find a way to get it and keep it.
Welcome & best of luck man!