will I have two accounts (one for each slingbox)
Nope. One account, two entries in the Slingbox Directory... unless you would prefer to have two accounts and choose to start a new one for your Pro HD. It'll work either way. You can add/remove as many Slingboxes as you like to one account in the Directory.
Have a look here for additional information on Sling Accounts:
As for your Dish DVR, the Pro HD should handle that as a video source the same way your AV did. Setup procedures should be very similar too. You mentioned HD... I recommend that you avoid HDMI when you hook up the Pro HD. It has a tendency to cause signal loss headaches due to the HDCP copy protection scheme built into the HDMI spec. Use component cabling to the TV instead.
I don't understand what you're talking about in regards to the wireless camera. If it behaves as a source that was compatible with the AV, it should work with the Pro HD.
Hope this helps !