STARDAR1,
I hope this provides some clarification.
What you are referring to as "O 27" is typically the smaller. It's also sometimes referred to as "Traditional". "O" (also called "Scale") refers to anything that is 1/48th the size of the real life equipment.
Most anything that is labeled "O" or "O 27" can be run on O Gauge 3 rail track. The big difference between the 2 is the size and that can limit if it can be run or not. That limit is caused by and related to the diameter of the curves. O 27 can be run on pretty much anything (the "27" refers to the diameter of the curve - 27 inch diameter). "O Scale" will typically be more intolerant of tighter curves and will derail if the curve is not wide enough. If you look at a Lionel signature catalog for example. Most of the locomotives have a "minimum curve" in the description (the SD45 in the newest catalog has listed a minimum curve 054...that means the smallest track curve needs to be 054 (54 inch diameter)). Looking at the list of things you have inherited, you shouldn't have any issues.
One suggestion...don't sell anything quite yet. Wait until you have had an opportunity to see stuff running and then decide. Like people have mentioned, it's all personal preference. If you like it, keep it. If not sell it. The only important detail is to have fun.
Oh...and ps. Welcome to the hobby!!!