I've got a variety of Dept 56 buildings in different areas of the layout. North Pole stuff for the North Pole, naturally. Christmas in the City buildings for a section of the layout known as Irishville (lots of pubs), and Snow Village buildings/houses for the other sections. The scale is different for each so grouping them together and having separation of the sections is key to a tolerable perspective. All told, 125 or so structures.
One downside to the Dept 56 buildings is you aren't going to have inside scenes unless it's already part of the building. There aren't windows to look through or rooms with floors on most of the buildings. So all your people action is out in front of the house, or in the backyard. Another is the buildings are mostly winterized - i.e. snow somewhere on the roof or base. I just look past it and still have people swimming in a pool, mowing grass, having a cook out, playing baseball, etc. My wife has a more difficult time blocking out the snow and can't understand how I have been able to get the roses on a vine to bloom on the same street that Public Works is plowing snow.
I started replacing the original lighting wire and bulb with LED bulbs, but then went in another direction altogether to get rid of the wires and plugs. I bought the Woodland Scenic Just Plug light hubs and daisy-chained those together and used a stick on LED in each of the buildings. Easy to hide the LED wire as they are small and can fit in any hole under the building. Also doesn't require you to drill big holes in the layout to get a light cord through, or have a 125 sockets to plug into. Used 40% coupons at Hobby Lobby to save money and replace a few lights a little at a time.
I bought my buildings used from different venues, but it also took some time - Goodwill store, ebay, local auction, garage sales. Some of the stuff you see for sale online is way overpriced the closer you are to Christmas. There is a site called Find My Department 56 that will give you an idea of what a particular piece has sold for recently.
Anyway, a few ideas.