If you date your cameras, you marry your lenses. That's because, unlike digital cameras, a well-chosen lens can serve you for a very long time.
I still have one lens that I bought thirty two years ago. And I bought it used. I doubt that will be the case with any of my digital cameras, ever.
In the past, I was a lens speed freak and was willing to spend great sums of money to have very fast glass. I now realize that lust was misplaced. If I had it to do over again (and I do, and have) I would lean more on reasonably fast primes and here's why.
Moderately fast primes are (much) lighter, (much) cheaper and often just as sharp (or sharper) than their speedy siblings. For Nikon shooters, the Nikon 28, 50 and 85 f/1.8 trio of lenses are great examples of this. They weigh next to nothing in my bag and offer great performance. Also, I have moved away from primarily using fast zooms. Rather than a fast 24-70/2.8, I'd now opt for a trio of fast-ish primes and a decent, slower zoom to back them up.
This way, you get a stop (plus) faster at each focal length, backups throughout the 24-70mm range and you lose the most daunting aspect of the speed zoom: an expensive single point of failure.
In general, remember this when it comes to ultra-fast DSLR lenses: you pay through the nose for them when you buy them. And then you pay again, in weight, every time you lug them around. Remember that cameras have amazing high-ISO performance these days. And they are just going to get better as we go.
As for my Fuji lenses, it is pretty hard to go wrong with their primes (although I'm not a huge fan of the 60mm macro…) But the others are small, gorgeous and fast. I love the built-in 23mm (35 equiv.) of the X100s. It's sharp wide open and it has beautiful flare when you point it right into the sun as seen above. It's my most-used lens. Which is a good thing, given that it's welded to X100s.
Painted Hall, Greenwich, UK with Fuji 14mm/2.8
Along with that, the 35/1.4 (50mm equiv., seen at top on left) and 14/2.8 (21mm equiv., on right) are my go-to lenses on the interchangeable lens Fuji bodies. If I am shooting tight headshots, maybe the 56/1.2 in the middle.
As a backup for the lot, I like the 18-55mm f/2.8-4.0 kit zoom (not shown). Which, although not a speed lens, is great optically and has stabilization.
Actor Ben Lurye, with Fuji 18-55 f/2.8-4
Good performance from a kit lens (the zoom that often comes packaged with a camera) is not a given, as many of them are crap. If you are using a kit zoom as your main lens, know that it will be be sharpest near the middle of the aperture scale—let's say around f/8. In fact, most lenses are great at f/8.
If you like to hang out close to wide open, grab a (used, if necessary) prime at your most-used focal length. You'll be a happy camper—and have a backup if needed.
What to buy? And when? That should be driven by what/how you shoot.
Here's my thinking on lens progression, driven by how I shoot. Most of the time I am going to go out with a single, prime lens. I like the size, the weight, the speed and sharpness. Actually, I also like the discipline of having a single focal length. It helps me to see better.
With a new system I'd build out my wide/normal/short-tele primes, then get a decent wide- to short-tele zoom as a backup or for times when I would want one-lens variability. Only then do I start going for more exotic lengths if needed.
The reason is simple: most of my work will be done with the bread-and-butter focal lengths of moderate wide to short tele. So I want good quality, speed and backup in those lengths before I start to get crazy with a superwide or whatever.
You can easily try out a lens for no risk. Buy a good example used, from someone with a good return policy. If the lens is a dog, return it immediately. If it is good enough to where you want to keep it and play, you can always resell the used lens for about what you paid for it within a year or two.
So, very little risk. (In fact, if I was going to need to rent a lens for more than a week I'd do it this way. Your rental would be practically free.
And obviously, if you love a lens and decide to marry it and keep it forever that's best possible case.