Found answer by readin a couple of pages back!

"Why are you replacing the sashes? What are your existing windows? A well maintained original wood window with a good storm gives the same or better thermal performance as a modern replacement. Well maintained is the key phrase." I have to agree w/Patser - I realize not everyone does - I just paid around $190/double hung window to have new parting strips, glazing replaced on many, re-balance (this guy was meticulous there) - entire window rehab. Here's a couple of issues: 1. Typically the best suited window for your home is the original - that is the "custom" one. 2. from an esthetic standpt - I rarely see an older home with new replacement windows and say "wow look at how those windows made a difference" - I think many people don't really take a look at their homes outside & in to determine the detail in old windows - They are sold strictly by the ease of use & energy efficiency - Not realizing the authentic divided lights has much more to offer. 3. Many of the "energy efficient" windows are not without flaws - like the seal breaking or the window being improperly installed (another issue, yes) 4. dh is a banker so he tried to look at it from an investment (we all do) - in any case he tried doing the math - we have 26 windows - so lets say that came to around 20K to do a decent replacement or sash unit (i.e.Marvin) - and you received a return of 5% a yr on energy savings - lets say heating season runs $4,500 that is a return of $225/yr - we don't have central air DH doesn't think he live long enough to recieve a return on his investment - even a 10% return My $.02

So the doors open in to the space... Would you be opposed to removing them? Sounds like this may be more of a relaxing space to hang out and play on the computer, than a true "work from home" office. You'll lose square footage if you do a built-in, but you could put one on the north wall. Float the desk in front of it facing the south wall, and angle a chaise at the window facing the view.

Before you replace the ceiling, have someone investigate those joists. They should be continuous from wall to wall and not be sistered without vertical support. Those laps make me very suspicious you may not have that upper level properly supported. If this is the case and you have bounce in them, that is likely what brought your plaster down. You may need to build a temporary wall to support them until an engineer can advise as to what kind of remedy you need. Likely you’ll need a beam up in there, but it will also need vertical support that runs down to footings. Even in older homes it is very unusual for ceilings to fall without obvious cause. Good luck.