Yes, that external case will work just fine.
Yes, it would be a good upgrade, but, considering your current circumstances (you sound like you are in a rush), it would be best to forgo the dvd burner and get back to it at a later time. Or get an external burner, or you can make a trip to Fry's and see if they have anything. Most retailers do no sell notebook replacement drives. They mostly sell internal desktop burners or external burners.
If weight is a concern, then I would skip HP.
Sonys are like the Mac laptops. Lots of power and the price to match it. They are lighter than HP, and if you have to carry it back and forth, Sony would be a good choice.
If you want to do CAD and image editing, the XPS M1210 would be a better choice since you can change the video card. The Intel 950 is fine for images, but CAD would need something a bit better. The Sony does have a bigger display which would be better for work. I would go with the Sony if I could cut back on the CAD work. If you can't cut that, then the Dell would be the way to go. I would have an issue with the Dell since it does have a small screen. That might not be the best for image editing.