I too am interested in dogs for protection and some
very experienced guys have already responded.
Some high tech devices might be in order.
Since your wife cannot see, install some high quality
microphones, placed at key areas. Add some subtle
noise makers at these areas. For example, gravel for
a walkway, squeaky gate, etc. Then she can practice
listening as friendlies pass thru the areas. Along these
same lines, eliminate natural noises that are too loud,
like wind whipped branches that slap the house.
I once saw siesmic detectors (4) that transmitted on
a vhs air frequency, militay surplus, real neat, with a
cheapo radio reciever. The 4 were made to be portable
and each had an antenna that looked like a 18inch blade
of grass. Doing a web search, you could probably find
some info on simple DIY siesmic detectors. A call to a
local university geology department, might result in some
some helpful info.
Pressure mat switches and electric eye beams might be
useful. Other swiches could be pressed into service, such
as those used in electric sport fencing sword tips.
Infrared detectors could also be useful. They could be
used at both high levels (human level) and lower levels
(dogs, cats,foxes).
Lastly, consider cameras, that you or a friend can view,
from a remote location. Some means of recording the
camera views is important. Many rapists will case the
place by peeping, before they ever try an entry.
Web cams are getting cheaper all the time,
but there might be higher quality ones also.
A camera positioned near the entry of your driveway,
could pick up a car license: invaluable.
All of the above could be completely hidden from an
intruder or any casual visitor. External infrared-activated
lights would be noticed by anyone, but are probably
worth having, especially if a camera takes a picture.
These are cheap at big box stores.
Keep your car in a closed garage; why make it obvious
that you are gone or at home.