why would one pick a system based on a lens?
Lenses, for the most part, transfer from camera to camera and don’t depreciate significantly. Digital camera bodys, on the other hand, are almost fully depreciated in 4-5 years. So, it makes sense to buy quality lenses and cheap bodies if you can’t afford quality all the way around. You will get better photo’s that way also (depending on your skill).
So, the first step should be deciding what you need in a lense. You can bid on a used lense on Ebay and pretty much count on getting your money back out of it at any time…which is better than my mutual funds. Unfortunately, it is hard for beginners to know what lense is best for them. Often they are seduced by “mega-zoom” lenses, or else they just go with whatever cheap lense came in the “kit” they purchased. The lenses I use the most are my 50mm F1.8, my 28mm F3.5, and my 135mm F2.8. They are all primes (non-zoom) which generally gives you smaller, cheaper, and better than a zoom for any given focal length (but you may have to carry several lenses). I have a 200mm prime and 85-300mm zoom also, but seldom use them. My next lense would probably be a 24mm or 21mm, and these (Zuiko glass) sell at established prices that haven’t changed much in years. These are all full-frame 35mm lenses.
You NEED a fast lense (F2.8 or less) for indoor photos without flash, or for control over depth of field. You might want image stabilization, a fast/quiet autofocus, compatibility with a set of filters you have. You might want to go with a slower lense if it will be taken on hikes, etc, where weight is a concern and lighting is not a problem. Do you want to be able to upgrade to a larger/full frame sensor at some point? Then you need a lense that is compatible.
There are a lot of choices in any lense family…Nikon, Canon, Olympus, Sony, etc. Over the years you will tend to aquire lenses and other kit that lock you into a family of bodies. But for the most part, the differences in bodies are not major at any given price point, especially Canon vs Nikon…you can’t go wrong with either of those.
Save some budget for a high quality (Gitzo) tripod. It will do more to improve your photo’s than double that money spent on lenses or bodies.