Hey Richard,
The great news about antique houses is they aren't making anymore and the supply is somewhat limited. The bad news is that not a lot of people want antique homes due to the issues you mentioned, like maintenance, etc. I live in NH in a home built in 1760. We bought the house from my folks 2 years ago. Previously we lived in the house next door, built in 1766 (Both homes were built by brothers originally). So I've had some experience buying, selling, updating and maintaining antique homes.
To some degree, the antique will be compared to others for the normal stats like sq ft, bedrooms, bathrooms, etc etc. I know in our dealings with old homes the appraisers and banks had to open up their search area when developing comps. They weren't always compared to homes of the same age, but mostly homes 100+ yrs old.
Perhaps the biggest issue when rehabbing/renovating an old home is what you find when you start taking things apart, or even sometimes what you don't find! Have you had the ability to walk through the home with a contractor yet? Have you rehabbed a home of this age before?
I think the barn/garage can add a lot of value for the right person. I suppose it all depends on the condition of the out buildings and if they have utilities. Our barn has a work shop/tool room, gym and wine cellar. We've also converted the lower level to vehicle/motorcycle storage in the winter to generate extra income. So there's plenty of uses for those buildings for the right person in the right condition. It probably just needs to be presented that way when trying to establish a value.
When we sold our first antique it took less than a week. We priced it well and it helps that we're in a desirable town in southern NH (Hollis). Ironically we only had one bidder, but they had been looking for an antique for well over a year in southern NH and they weren't letting our place go!
If I had to decide whether to invest in a flip that was more up to date vs. antique I would go with a newer house only for the easy of doing business (updating, flipping). If time and money aren't a concern to you, the antique could be a fun option, but be prepared for some bumps along the way and for it to take a while.
Best of luck!