The answers to your questions are:

1: No

2: Maybe

3: No

I don't have a T7500.  But I am working on a T7600 right now so I think most of this will apply.

Are you using the onboard SAS controller or have you installed a PERC Controller?

For the onboard controller you press Ctrl-I when it prompts you on screen.  Setting up a raid 1 array in there is very simple.  You should see all 4 of your drives, so you just select new array and then select the two new drives.  The whole thing just takes a few seconds.  After the system reboots and windows starts up go into hard drive manager (computer->manage->disk management) and format the new drive.

If  you have a PERC controller, you'll press Ctrl-R when prompted during boot up.  Then you'll create a new Virtual disk with the new hard disks.  You'll also need to initialize them while you are in there, and that will take a while.

I'm not sure, but you might need to format the drive once you are in windows.  But once you open Windows Explorerer, if you don't see the new drive then you'll need to go into the hard drive manager and format it.