If you are going to use the grist hopper above the mashtun (that's what I would do), you can be a little more flexible with your mill/auger size. You would also save the expense of a 2nd auger, not to mention one less piece of equipment to malfunction/maintain. If you were going to mill into the hopper and then auger straight into the mashtun, you would want to size the mill/auger to provide you with a flow of about 25-30 lbs per minute to acquire a mash-in time of 15-20 minutes. However, you would probably have to build a platform to mount the mill above the hopper. On a 7bbl system you can also get away with skipping the hopper altogether and milling/augering directly to the tun; you have to run back and forth some to reload each bag, but it can be done.
Components that have worked for us:
Apollo in Canada makes a good affordable 2-roller mill that would work well for a 7bbl system. We have a larger model that we are very happy with. However, you will have to custom fabricate an adapter for a flex auger boot with an Apollo mill.
We have used several Brock Chore-Time flex augers. A Model 75 (2 1/2") will deliver approx. 25 lbs of malt per minute over a short, relatively straight run.
I would think you would want a grist hydrator regardless of which method you used.
A brewing equipment broker can help you select the components that are right for your system if you are buying from them. I have purchased from Sound Brewing Systems in the past with good success, I am sure there are others out there who can help you as well. You will definitely save money milling your own grain, have a larger variety of ingredients to choose from, and have greater control over the brewing process: all positives in my opinion. Good Luck!