Home office: Perhaps the full-wall shelving unit goes on the wall between the new office and current office? Then the window wall is left without built-ins around the window and the adjoining wall is open for artwork. It would look bigger.
Downright Simple: This is my DIY Built In / Wall Unit made for 60" TV. I used three in stock brown maple Home Depot upper kitchen cabinets (30" wide x 18" high x 12" deep), plywood, bead board, 1x2 & 1x3 boards and some decorative moulding! The cabinets required three coats of primer and five coats of paint. I used Olympic Interior Latex paint in Satin finish. The color is "Crumb Cookie".