Smoothing out the wall texture with a plaster coating is called "skim coating" and can be done in your spare time using premixed wallboard mud from the hardware store. You mix it, possibly thinning a little, and apply with a wide taping knife. When dry, you can smooth it by lightly sanding. Then prime and paint.
But, sometimes a textured wall can be a nice touch, especially if the trim and other details are clean and simple. Widows you can totally do but if you've never done it before the learning curve will feel steep.
I'm also a fan of open plan homes. My home's main floor has only 2 rooms, a bath and a den/tv/guest room. the living, dining and kitchen are all open to each other. This makes hosting parties simply a matter of moving furniture around as needed.
To open up small spaces, think about lightening up paint colors, and windows. Home surgery to consider is wall removal like you are already thinking, and doorway enlargement. Doors that are seldom/never closed can be removed, and doorways enlarged to 1.5-2x their former size. Windows and skylights add natural light and potential views.
