"And how do beginner road bikes differ from more advanced road bikes? How much money is it typical to spend on a good quality bike? I want to get a bike that will last me, not one that is just going to break and then force me to buy another one. I just want to buy one bike and use it. "
The differences in beginner vs more advanced...
Beginner bikes are usually heavier (25 pound range) where more advanced bikes are usually under 20 pounds.
Beginner bikes have lower quality shifters, cranks, pedals and derailleurs than more advanced bikes. As a 235 pounder, the brakes on most beginner bikes will be sketchy on a fast descent down a steep hill. Many can perform well with better pads, but if the bike is under $800, odds are, the brakes will not be adequate.
Beginner bikes usually have a brick for a seat. There are exceptions, but a good seat can cost you $100.
I'd say to get a durable bike that you will be happy with, you will be in the $800 range.
As far as back pain, that is a matter of core strength and fitness. Some road bikes are relaxed fit (sit you up higher) but as your fitness increases, you will find it tough to adjust these to an agressive riding position.
As far as your weight, any reputable road bike will be fine.