As for the monitor, depends how much you want to spend and how much style you want. The apple monitors are very sexy but are more expensive than say a dell or other monitor maker.
As for whether you should buy another pc depends on if 1. will both of you be fighting for use of the mbp if you decide to not buy a desktop? 2. is the mbp always at home or do you take it on business travel or to work?
If you are always going to competing for the one computer in the house, or if you take the mbp with you often , I would suggest buying a desktop so everyone is happy.