Gen 3 Lib FTW, I personnally think they are a great looking car, just a little bit agressive.
Another thing to look for, although rare, is the connector to the fuel pump/level sensor assembly overheating and melting. Symptoms are simply not starting intermittently.
I've seen three gen 3's now with that problem, expensive repair as the assembly is $900 odd new.
Dougie's right about heater core issues, they do leak and can block up if the coolant isn't maintained properly, and its very labour intensive to replace them.
I would avoid Gen 3 H6 Outbacks, they have an issue with catalytic convertor failure now they're getting on.
The only way to resolve it is to replace the front exhaust with new, genuine parts, up to $5000 just in parts.
But, good choice of model I say.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i