As far as the keeping a Subaru for a long time, there are lots running around with well over 200,000 kms on them with no issues. Also, as its a boxer engine, I think its just prudent to use the conditioner. The bottom part of those gaskets have to sit in coolant whenever the motor is off, so any preventative measures that can be done can't hurt.
When my chums and I get together, it looks like the Subaru club has come to to town. It's the favourite brand among climbers and skiers, and we're also cheap, so we keep 'em until they are completely worn out. My nearing 250K 2001 Forester is one of the lowest mileage Scoobs in our group, ranging from ~190K to close to 500K. 350-400K is typical. Some have had headgaskets replaced, some haven't.
Not driving your Subaru doesn't "preserve" it. People yelp about their 7-10 year old Subaru with low mileage having some problem. These cars age rapidly if they sit. They also despise shirt trips where the fluids don't get super warm. Most owners of low mileage cars have all the same problems as very high mileage owners, maybe more, but they don't "get their money's worth" out their car.
The good:
*Subaru bodies/chassis are very rust resistant.
*A/C systems are robust and not bad to repair if something does happen.
*Most electrical systems are good for the life of the car. ECMs and power accessories are trouble free.
*Suspension parts are pretty good. My front struts were changed at about 180K, one of the rears is sweating now. My Forester is used off road and on some awful roads, and I was surprised the parts lasted this long.
*Automatic transmissions are robust. There is an internal seal that gives out eventually, requiring a teardown. If the AT is sluggish to go into forward gear from neutral, that is the sign. Mine is just showing signs of this, but you can greatly extend the life by changing fluid often and waiting for 2-3 minutes before driving away, allowing the trans fluid to warm a bit and not shock the piston seal with cold fluid.
* AWD systems are very reliable. The AT version has no center differential, and is perhaps a little more long lived than the MT versions.
* Front and rear diffs and other mechanical bits are strong.
* The bottom ends of the engines will outlive the owner if the oil is changed every 6-8K.
The bad:
*DOHC 2.5L engines are terrible. Don't buy a pre 2000 2.5L car!! The SOHC 2.5L engines still suffer head gasket failures, but they're external leaks 99% of the time, so not a catastrophic failure like the DOHC engines.
* Rear wheel bearing failure is common. Mine are original, touching wood.
* Parts, when needed, are expensive. Subaru dealers seem like con artists for the most part, with many people dissatisfied with their dealers.
* Thanks to high body parts cost, insurance can be expensive. Our 2001 Forester costs significantly more to ensure than our 2009 Corolla. Double!
