@Shane Clark I have a two-family and a three-family - both built in 1913 so I have dealt with a lot of issues. I will say that I am a project manager for a general contractor so I knew what I was getting into. At times it can be overwhelming depending on how much remodeling you end up doing.
1. Foundation - Definitely look for signs of water in the basement - always good to look on very rainy days. Also, check for cracks and bowing of the walls as @Nick Noon said. In any old home the boiler/furnace, hot water tanks and washer/dryers should be on blocks as they all tend to get at least some water. Check to see if there is a sump pump. When I had the home inspection for my two-family there was an open sewage drain on the floor of the foundation. I ended up having it closed but there was a week of torrential rain and I wasn't living there at the time because I was doing a lot of remodeling to my unit. When I went I had 6" of water in my basement! The first solution is to install gutters and try to get the water away from the foundation before investing in other expensive options like french drains, etc. There were not enough gutters on my two-family and the gutter that was on the back of the home was depositing under my deck right in the problem area of the foundation. With the proper positioning of the gutters and downspouts I have virtually zero water in my basement - only a trickle on extremely heavy rains. I did, however, install a sump pump just in case.
2. Electrical - When they say the electrical is "updated" it could be the electrical panels and any visible wires (attic/basement) but there will almost always be knob and tube in the walls and ceilings of the units. Check to see that there is a panel for each unit and the a separate landlord unit.
3. Plumbing - Check to see how old the boilers/furnaces and hot water tanks are. Have they been serviced? Are they oil or gas? If they are oil do you have the option to convert to gas? The gas line for my two-family ended in the middle of my home so I was the last house on the street that is able to convert. Most plumbing in very old homes is cast iron and it will eventually leak. Some of the water lines are brass and they will eventually leak as well. Check to see if there are asbestos wraps on the pipes in the basement. If there is have it in your offer to have it removed and have a paid receipt from a licensed asbestos abatement company.
4. Roof - Most people do not know how old there roof is. Check the attic and to see if there are any water stains on the boards and check the ceilings of the top unit. A big thing in very old homes is how wide is the spacing between the old plank boards. If there are big gaps you will have install plywood on the whole roof before shingling. This can get pricey.
5. Siding - Most older homes have either wood or asbestos siding. Asbestos can last a very long time but you have to be careful if any of the shingles are cracked or falling off. Wood can be expensive to paint and the maintenance can be higher. If it's vinyl there will be lower maintenance but check to see that all of the windows are capped and that they are caulked properly so that no wood is showing.
6. Windows - Check to see if the windows have been updated. Some very old home still have single pane windows. If the windows are updated are the frames very small? A lot of times they are cheap windows so they balance systems can break, etc.
7. Insulation - Most older homes lack insulation. This can significantly increase the heating bill (and cooling if the home has A/C). Check the attic to see if there is insulation. Are the outside bays of the basement insulated or the whole ceiling in the basement? Unlikely that there is much insulation in the walls.
8. Plaster - Most old homes have horse hair plaster. If you start doing work by taking off trim or demoing a part of a room the plaster tends to crumble. If you are not doing much and there are patches that are missing there is screening and then you can skimcoat the area to seal it. If the wall is weak they have buttons that suck in the plaster. Usually the walls in old homes need some work. Check to see if the walls are painted or have wallpaper. I absolutely hate wallpaper. It is such a pain to get off - especially in older homes because they usually have multiple layers or have painted over the liner or wallpaper. By stripping it you tend to damage the walls a little bit. You could follow previous landlords and wallpaper over though.
9. Woodwork - Is the trim in the home painted baseboards, window sills, etc.? This will likely be lead paint. You have to be careful with this if a renter has children. There are ways to eliminate surfaces that a child could bite such as capping the baseboards with a piece of half round.
Since you are in Massachusetts if the home has National Grid or Eversource there are incentives to insulate the home through the Mass Save program. You would need to do an energy audit and the auditor will give you recommendations (cost breakdown) on improving the energy efficiency of the home. They divide what will be paid through the incentive and the remaining costs to you. The company I work for is one of the installation contractors. It is a good program as you can save money and improve the energy efficiency (and value) of your home. They also have a HEAT loan where if you qualify you can get a 0% loan to replace a boiler/furnace and hot water tank, and windows/doors. I ended up using the program to replace a boiler. The only way to beat 0% is free so it's a great thing to take advantage of.
I'm sure I left out things but those are the main things I would look for. The two-family I live in and I ended up remodeling my unit that I live in. The cost ended up being much higher than I originally intended because once I started remodeling one room and saw how the old horsehair plaster started crumbling I kept going. I ended up gutting a lot of walls. When you do this then the knob and tube wiring is exposed. Need to get that replaced and then insulate/drywall/plaster. Then I opened up a wall and changed the layout, creating an open concept kitchen/family room. In the process plumbing needed to be moved. It's all how far you want to go. If you start remodeling as I did you tend to open a can of worms with old homes. However, I am extremely happy with everything that I have done and when I eventually move out the unit will demand rent at the top of the market. Plus I know everything is new so I will have less big issues that come up each year (at least for that unit).
Best of Luck!
John
