Need Help On Purchase

MBP with cinema display, or use thunderbolt to connect it to your parent's iMac.

 

If money is no object...

Agreed, if you the budget to either buy a nice used car, or an insanely huge T.V.? Go with the MBP and the ThunderBolt display. They are truly amazing.

If you do a bit of searching on the Mac Setups discussion you will find there are many competitive monitors out there for far less cost. Now, the coviate I found in your post; LightRoom 4. While I have never owned an air, I can tell you the photo editing that I do on my road warrior at times can really be a P.I.A. and that is with a pretty nice 15" screen, so with either a 13" or 11"? Ugh, I think I would either need a magnifying glass or to promptly shoot myself in the face. That could simply be me.

So as I see it, there are a few ways you can go. Unless you macbook is having significant issues and truly needs to be replaced, you can max ram, add an a SSD to really speed things up for a fraction of the cost of a new mac.
If you are pining to upgrade to a shinny new mac? Already stated: Go with a pro. If you have the bread? A 15" would fit the bill. With the access to your parents mac, there seems to be little reason to spend anything on an external display until you really need it. In fact if you are a student, you can get some shweet discounts at the apple store, or find a nice deal on eBay.