Mike you can add Sirrius or XM to your factory radio I might even have one you can have.
You can also Bluetooth to a factory radio as well in most cases.
If you want a new head unit remember or style Ford vans are really DIN and 1/2 size. So a single DIN radio will fit with a 1/2 DIN storage box/installation kit or you can trim the dash penal a bit and use a double DIN radio instll kit. I really like the American International Double DIN kits as I think they fit the best of the two to three I have tried in Ford trucks and vans.
http://www.amazon.com/Ford-Double-Din-D ... mount+ford
You will also want to buy a speaker and power wire adapter just to make it easier.
http://www.amazon.com/Metra-70-1771-Har ... =pd_cp_e_0
And DIN radio removal tools for your factory radio although you can borrow mine since you are onlly going to be removing you old radio one time.
If you dont care about Navigation or a back up screen you could easily go witha single DIN radio. Some are a bit complex as the are smaller and some are not touch screen.
This was a good thread about speakers and head units.
viewtopic.php?f=14&t=9720&hilit=stereo
Good luck and please let me know if I can help...
...Jamie