Who is made of Money these days? If you are, please come on down to the store and buy one of every thing we have!
I am of the opinion that to make work easier you must have the right tools. In this industry if you are working, having the right tools makes your life shooting much much easier. Not all of us can rush out and buy all the best toys and tools for out kits. We for the most part have to work at it and build it up gradually. Case in point. Both Nikon and Canon have fast zooms. One in particular is the 24-70 f2.8 range. C and N have versions and you will have to hand over between $1900-$2300. In most cases you will be extremely happy with that purchase.
What are my choices if I don’t have 2 Grand? You could start with the Sigma 24-70 f2.8. Reading any review in forums online you will read lots of things good and bad. But in the end i think it equals it out, puts you right on the fence. Priced at $899 it will get you going and you will have all the benefits of what that focal length has. You will see an immediate increase of quality in your images. It will distinguish your work from most of your competition. Use it until you have the the big bucks saved up to get the Nikon or Canon version. Hey, you could even trade it towards any of your dream lenses on your list.