I'm used to houses with a forced air furnace in the basement and an AC compressor/condenser unit outside. Now I'm looking at a house with an exterior package unit with gas heat and AC and fan motor all in one unit. How good are those things? How durable? Efficient? What are the tips and tricks for maintaining & repairing them? What to watch out for? In my experience, an old-school gas furnace is dead simple, and can last forever with repairs. Nat so much for gas pack units, I suppose?

I have three houses with gas packs.  One is a York system installed in 1997.  The only things that have been done to it are a controller and an igniter.  The second house has a Comfortmaker unit installed in 2004.  It had a fan installed once.  Last house has a Comfortmaker unit installed in 2003.  It has had a burn chamber (I believe that's what it's called) and a controller.  
The heat is realay good, and the AC has been good and reliable.

Tip for you is when colder weather comes and you turn it to heat for the first time in awhile, you may hear the unit trying to come on, but you get no heat.  Before calling for service, remove the service door.  Find the little silver thing that looks like a puck.  It will have a little tube going to it.  Pull the tube from it, wet it with spit or whatever, and stick it back on.  The unit should then light and blow heat.  That little tube has a tendency to break it's seal.  I really don't know why they didn't have a clip to hold it tight.

How bad is this? (I'm looking at how old it is)

27 years old?

That's pretty old... maintenance will be more and it won't be as efficient.

That being said, it allows you to upgrade to something YOU want if/when it does fail. Just account for it in your budget.

MFD 3/89  that is the built date