Buying an older (or as some might deem it if the residence is old enough: historic) is something for the ambitious and very possibly well-funded prospectors.  I admit, I have a fascination and great love of old houses, especially built in the 1920's or older.  However, here is some advice to consider if you are deciding whether or not to purchase such a piece of property:

1-Check the foundation.  Admittedly, houses built in the old days are generally built better than today's but b/c of their age, you'd be much better off checking this one thing first if you looked at nothing else.  A home with a cracked or otherwise damaged foundation is a very large cost that many homeseekers would not find worthwhile to invest. Cracked or badly damaged foundations can happen from the likes of  bad drainage due to softer soils or poor landscaping--such as houses where the land in front or back slopes toward the house).  This may cost thousands of dollars to repair.  So, if you think you are getting that old house for a really good deal, make sure you don't have to throw in an extra $10,000 to fix the foundation.

2-Check the wood under and around the house.  If you see wood that appears chewed, shredding or flaking, this could be a sign of termites.  Hopefully a home inspector would catch this but no guarantees.  If you are not sure, a highly recommended Pest inspector would be your best bet.  Don't just rely on the one recommended by the seller's agent unless you have good reports about the inspector.  It is not hard to find flaky inspectors who want to make a fast buck.

3-Check for lead-based paint.  Lead was a common ingredient in paints in the olden days (in fact, if the house if pre-1980 you should check just in case).  Lead is known to cause impaired learning in children exposed to it.  Of course, this most often happens if the paint is flaking off and the chips are ingested but if you plan on having children or have them in this old house, it would be good to look into it for their well-being and yours.  Please visit the EPA (Environmental Protection Agency) website to further inform you of the hazards of  lead. 

4-Have the house tested for Radon.  Radon is a naturally occuring gas in the earth.  It can seep in from under your foundation.  It is one of leading causes of lung cancer next to smoking.   You can purchase Radon-kits from your local hardware chains such as Lowe's, HomeDepot, or similar stores.  Most times, they (the company who created the kit) will charge you in addition to have the sample you collect tested at a lab but peace of mind is worth it.  You can also have a certified Radon inspector check for you.  This is one of the only points where you could possibly do it yourself.

4-Check the plumbing.  I am no expert on this but plumbing can't last forever.  Ok, it could last a long time but a hundred years?  Even 50 might be questionable.  One thing for sure, galvanized steel is the worst as far as pipes go.  Esp. if they were lead sautered (possible health hazard b/c the lead can leach into your water system--see comment on lead-based paint) Not only that but many homes back then had PolyButalene piping in places instead of the PolyVinylChloride (PVC) piping we use today.  Now, some people will swear to you that the PB pipes are just fine, still intact.  And they may not be lying to you.  The problem is, PB pipes are tempermental in a way:  in many places they have degraded, causing lots of woe to the homeowner (and this includes your water heater, which you should also check).  But there are the few folks that have had nothing happen.  But let me stress to you the word few.  No one wants to come home and find their water heater spewed a ton water on their carpet (or wherever it is) b/c the pipes busted.  Hey, it happened to me.  And just guess what type of pipes the water heater had....!

5-Finally, Check out the roofing and insulation.  Namely looking for Asbestos here.  Asbestos was commonly used in roofing materials (including insulation) up until around the 1970's when they found it to be a carcinogen (that means cancer causing).  Now here is the thing:  it is only a danger if the material is frayed and loose, kind of like cotton candy pulled apart.  The dust is the bad part. Other than that, it is not a  worry.  I, however, am a bit leery of the stuff so I am not sure I could feel at ease with it being around.  However, there are more materials than I named that use Asbestos as a component.  Do some searching on the internet to find other housing materials that have asbestos in them.  By the way, don't just go poking in stuff... have a certified contractor look at these things for you! (not just for this one but for all of the points!!)

Good luck in your search.  Lots of homework is the key.  And remember, you are your own best advocate!