I don't care about that mirror thing and as I don't care, I've never got a problem with it.
I've currently got two of them and had one before, which has been stolen.
And you don't have to use the cheapest lens, you can find, although of course you can. I've started with an EF 28-105, but also a very good one, isn't a waste, as you'll get better pictures with better lens, even with the 5D.
Recently I've been to Vienna and all three days I used just the 5D, mostly with the EF 50 mm f/1.8 STM or sometimes with the EF 16-35 mm f/4 IS USM attached and I didn't miss my 5D III.
I've created an flickr album with some pictures of that visit here:
https://www.flickr.com/photos/lenzmoser/albums/72157679794732063