How much maintenance should I budget for a historic home?
As someone with a relatively newer house (1930s), and have experience in dealing with issues of an 1800s home (my grandmother's), there are a few things to consider which can dramatically affect your annual expenses:

When (if ever) were the following replaced or upgraded:


Attic insulation
Wall insulation
Heating system
Air conditioning (if present)
Roof
Exterior painted
Septic system (if present)
Well pump (if present)
Plumbing
Electrical
Windows
Drainage
Fence
Deck


In my house, I had the seller convert all of the electrical outlets to three prong / grounded, but the cast iron pipes started giving up the ghost a few yers back and started springing pinhole leaks that required replumbing the whole basement.

I'm currently in the situation where I've been dragging my feet on getting the walls insulated; all of the places I've contacted want to cut a slot from the inside to inject the insulation, but I've got older wallpaper a room that I actually like, and plaster walls which I know are a pain the patch properly. (none wanted to pull off the top row of wooden siding and inject from the outside).  Lack of wall insulation and single-paned windows, combined with an old oil burner means that I'm easily paying 2-3x as much to heat my house in the winter as compared to similar sized houses in my area.

I wouldn't recommend anyone buy an older home unless they're prepared to work on it themselves.  You'll have odd things come up as things just start showing their age (eg, boards on the porch that lose strength from age), stuff that needs to be modernized (replace older bathroom fans or light fixtures).  I'd also recommend putting away money for a housing fund every month, so that should something significant need replacing, you've got a few thousand saved that you can tap into.  My neighbors recently had to replace their fuel oil tanks, which was a couple of thousand dollars; my mom had to have their septic tank dug up.  Most of my emergencies have been plumbing related.  You may not be able to do all of these items yourself, so will want to have enough of a cushion to call in professionals if necessary.

update: added 'fence' and 'deck' to the list ... my neighbor's fence had a section rip out in a high wind about a week or two ago (4x4 was set straight into the dirt, and had rotted at the base); had to redo my mom's deck last summer as it seems that whoever set it had dropped cinderblocks in a hole, then set the pillars on top, so it was only latterally supported by dirt).