I'm going step-by-step also. But I'm upgrading from a working 5.1 system, so mine is a little different.
1. I went to 7.1 with the addition of a pair of QS8s for the sides, and moved my existing surrounds to the rear.
2. Getting a VP150 soon to replace my center.
3. Another pair of QS8s for the rear.
4. EP500 to replace my existing sub before I get the M80s, because I think the M80s will actually reach lower than my current sub.
5. Pair of M80s.
If you listen mostly alone, and can sit in the sweet spot, you can easily put off getting the center until later. Until then make use of the phantom effect from your mains.
People say to make sure the timbre matches across the front. But I'm replacing my center quite a while before I get to the mains. That's mostly because the timbre of my Sony center is already very different from the mains, and it is rather unpleasing timbre at that. I want to get rid of that thing as soon as possible.
