When you say you don't have any window treatments, do you also mean you don't have curtains? That would be my first choice because they completely warm up a space. If your space is eclectic (and even if it's not), I find that you have to get curtains in your home and try it to see if it will work. There have been many times I was sure I loved a curtain and then it was terrible in the space. Conversely, really simple curtains made a huge difference.
Also think about your spaces. Are you needing room darkening/blackout curtains? Heavy curtains to help keep the heat/air in? Do you prefer to keep it light and airy?
You might also consider IKEA for blinds and shades- you could try one room or so and see what you think. Not as high end, of course, but might be a good first buy. Plus if you're self-installing, it can be a lot to try to do the whole house!
ETA: Also I like my curtains to go all the way to the top of the ceiling, so mine are the 95/96" curtains.