Okay, first thing first. For 2-Channel music, a sub is not really important but it would be nice for classical materials.
You'll be surprised how much a good sub will add to music, especially with Heritage speakers.
For Home Theathre, it is a necessity. I don't want to build one since I have no time and I don't want to spend an arm an a leg for it, should be under $750.
Do you care if it's used or does it need to be new?
The room is about 20' x 30' with two openings, one on the left side and one on the bottom right side. However, please correct me if I am wrong, but I think that no matter how the room look, (i.e. number of openings, large or small) the sub placement should not be a factor if I get the right sub.
The size of the room matters if you're planning on getting good coverage for the entire room. A room that big with those openings won't "pressurize" like a smaller room with no openings. A smaller sub will definitely be noticed but may not keep up with your mains at louder listening levels.