Beginners make two big mistakes when they are "planning to buy into a system."
- They think they need to buy everything they might need before they start
- They seem to think that lenses won't be available tomorrow, or next year
The best way to do this is to thoroughly research all the potential systems, read the reviews, study their lens catalogs, then decided which systems might be right for you. Then, try to handle the cameras and lenses if possible, to see if the ergonomics suits you.
When you have selected a system then just buy one camera and one lens.
Then use that camera and lens as much as possible. You will soon discover what you need next. You won't have to ask forum experts what to get next, you will find out for yourself.
You see, what you will need next is entirely based on your shooting style, your subject matter, and your budget. No one can tell you what your next lens should be. You have to find out for yourself. At least, if you do it this way, you won't be buying and selling lenses off because you changed your mind, or made a mistake. That can get pretty expensive!
Use that one camera and lens and if it serves all your needs then stop right there.
But if you find yourself:
- Wishing you had more reach
- Wishing you could go wider
- Wishing you could shoot better in low light
- Wishing you had a different zoom ratio, or a different focal length
- Wishing you had weather sealing or some other feature
...then that would be the right time to consider a second lens.
And all those lenses will be patiently waiting for you. Some will even get cheaper do to promotions, rebates or sales. And there might be new lenses created in the meantime.
You might also be able to get a real bargain by buying the lens you want used from someone "who made a mistake" and bought something they didn't need or like.
Remember this photo?

It took me seven years to assemble this kit. And it all started with one camera and one lens. An Olympus EP1 plus a very humble 17mm f/2.8 kit lens that my wife still uses. And she frequently gets better photos than I do with all my fancy cameras and lenses.

You have absolutely nothing to gain by buying everything up front. If you are patient, and wait to see what you really need, then you can save yourself a lot of money and effort.