"The only differences I can really see is that the touring bike may be set up with slightly larger tires steel frame vs aluminum. It looks sturdier overall, which makes sense, when you're carrying weight. Having the extra range of gears makes sense too. I guess my question is - is a road bike good enough for touring?" ......
Yes, it's good enough for touring, but touring (long distance + carrying stuff) will not be a simple thing, depending not only on how much weight you carry but also HOW you carry your stuff. A touring bike has a longer wheel base so it handles weight and is not likely to do the crazy (and dangerous) shaking and other contortions that often happen with a road bike when you try to carry weight, whether you go with panniers affixed to your bike or with a trailer. When touring, you COULD try carrying your stuff in a back pack, but -- whooee - you want to talk about setting yourself up for some long term discomfort?! So if you really are going to tour, get a touring bike, even though it's slower than a road bike.
That being said, I admit, I have BOTH a touring bike and a road bike. The touring bike I use for tours as well as my day to day around town biking, as in getting groceries, going to the library, commuting to work, going to the local watering hole, ETC.. It fits me, it's equipped with a built-in lighting system, all my bags fit it -- I love it. My road bike I use only when I'm out cycling with the weekend warriors and competitive cyclists, for whom cycling life is all about speed. It's not my thing, though it's a BLAST to go fast. And all that functional cycling makes speed easy-peasy. But I get the trade off between the two, and so should you.
"Also, whichever route I go, what should I look out for when buying used? I bought my 8.5DS new and already know I'm not going to again because I now live in an even smaller space than I did and I'm not going to be thoroughly maintaining it every ride."
Personally, I think the most crucial things in buying used are FIT and a good quality frame that suits your needs, because the rest of it will wear out and can be easily replaced. Best of luck!! I've never regretted getting my touring bike, and it's seen heavy use for 20+ years.