I've yet to invest but I may have some insight based on what I've learned here and what I've gathered from books and YouTube. One thing I will suggest is reading the book, "The Flipping Blueprint". It's very concise and goes into a ton of detail about your second question (even the part about Zillow's Zestimates). I've also just started reading "The Book on Flipping Houses" which is a bit longer and more extensive but seems great thus far and I would highly suggest it, it's also part of the BiggerPockets bookstore.
I'm in Columbus, OH and I'm sure all of the other people from Columbus will come out of the woodwork on this one since Central Ohio is extremely hot right now. Inventory here is insanely low and any house that is reasonably priced will go fast. I've heard similar things about Cinci and Cleveland but can't speak for Vermillion or Sandusky. With that being said, if you're seeing that recently sold houses (within the past 6 months) sat on the market for 6 months or more before being sold, it's likely your house would do the same. It's also possible that those houses were not updated and people are looking for no-hassle, move-in ready homes, so I'd make sure to look very closely at houses with the same features that you're anticipating for your rehabs since, like you said, there may be more or less of an appetite for nicely rehabbed homes in Vermillion than for a run down "fixer upper". You could always call up a realtor in the area and tell them you're interested in investing in the area and would like some insight into the market.
From what I've heard and read, do not rely on Zestimate numbers at all. Sometimes they're spot on and sometimes they're tens of thousands off. What I've read/heard is that you should take anything that isn't a hard piece of data with a giant grain of salt. If a wholesaler tells you an ARV, don't trust it, go find comps in the area that can give you cold hard facts to gauge from. If Zillow tells you a Zestimate, don't trust it, go find comps in the area that give you cold hard facts to gauge from. Basically, if a person or system tells you something and it's questionable where they got their numbers, don't trust it, go look up comps. Ideally, you'll want to look for houses on the same block, or very near, with very similar features such as beds, baths, square footage, whether it was rehabbed and how extensive, lot size, garage, etc... The ideal comp will also have sold within the past 30 days but if that's not available, look at the most recent beyond that. Hope this helps. I know ARV is your most important number since it determines your MAO, so getting it right is crucial. I've also heard the holding costs for a flip can be a killer if you're using hard money, so knowing how long your house will be on the market is insanely important.