OP,
Like some of the other posters here, I fall into the "I love old houses" group. I own a 1930s home. That said, I suspect you may be romanticizing the positive parts of life in an old house and underestimating the negative parts. Here are some thoughts off the top of my head:
--You will be shocked to learn what it costs to heat and cool an old house. Old houses were mostly cold and drafty in the Winter and hot and humid in the Summer. Insulation was close to nonexistent and windows and doors don't seal well. Temperatures were generally kept cool in the Winter and people wore sweaters. People sweated in the Summer. If you live in an old house you either learn to live with lower temperatures the way people did in the old days or you pay exorbitant utility bills. Old houses weren't built with the ducting required by central air conditioning in mind. If you want to install air conditioning, you either pay for massive amounts of ducting, high velocity, or mini splits. And the AC will never be as efficient as in a well-designed modern house. I pay insane utility bills in the coldest months of winter, and my house isn't all that big.
--Modern houses are standardized, but old houses are often not. If you need to replace a door or storm door you may find that you need custom sizes. This gets expensive very fast. Ask me how I know. All that cool original hardware (locks, doorknobs, etc) is incredibly difficult to repair or replace.
--Old houses are maintenance intensive. As just one example; all that exterior wood trim needs to be scraped and repainted on a regular basis. If those wood floors are original, they need to be refinished. This doesn't happen often, but when it does, it is expensive and messy.
--Charming old maintenance intensive houses often have maintenance intensive yards. I gave up on doing my own yard work very quickly. And it turns out that dry stack stone walls need to be maintained (who knew?)
--You often need specialist contractors. You can't just call any local hvac guy to look at a steam boiler and radiators. Regular roof guys don't know how to deal with slate tiles. And when contractors come and see your big expensive looking house, they often give you a higher quote because "he/she is rich and can afford it."
--Work that is often cheap and easy on a newer house is often much more expensive and complex on an old house. Ask me about installing ceiling fans and the associated wiring in plaster walls and ceilings.
--No matter how well built the house was, stuff wears out. My main outgoing sewer line broke (it was terracotta) and the repair was expensive. This is the kind of thing that no inspection will reveal, and that you just have to expect with a hundred-year-old house.
--You can't let maintenance slip. Small problems get worse (and more expensive) if you don't deal with them quickly. For example, having a slate roof means spending a couple of hundred dollars every year or two to have a roof specialist do an inspection and repair any issues before they become a much bigger problem. If you don't keep up with that maintenance intensive yard and flaking paint, your charming old house can become an eyesore very quickly.
--You have to furnish that big old house. Large amounts of space in a period house imply filling that space. Buying that that Tudor Revival and filling it with Ikea flat pack furniture or bookshelves made from milk crates and 1 x 8s is probably not the look you envision.
--Renting or selling an old house can be problematic. Most people don't want to deal with the maintenance and the inconveniences. And you really don't want a renter who isn't going to either do the maintenance or work closely with your property manager to make sure everything gets done.
OK, with all those negatives, I love my old house. People ooh and aah when they come to dinner parties. Workmen shake their heads and say "they don't build 'em like this anymore" (just before they hand me a bill that will cover their kid's next semester at college). But it didn't come into my life until we were pretty much FI. IMHO, a house is basically a place to live. Buying one that's bigger, older, more interesting, etc. falls into the category of a hobby. If you want to spend a lot of your time and money on your house as a hobby, go for it, but I advise you to be prepared for big utility and maintenance bills and to be mentally and financially prepared for an unexpected ten or twenty thousand dollar expense to happen a couple of times in the first few years.
The fact that you're asking about hidden expenses indicates that you already have a good idea that they're out there. Big old houses are money pits, and if you aren't prepared to do a lot of work yourself, you need to be prepared to pay someone else to do it. My advice is basically to keep your housing modest and invest the difference. Down the road, when you are confident you know where you want to live for the long term, you will likely be in a position to buy a charming old money pit if you still want it.
Good luck with whatever path you choose!