Been looking at foresters, one very close to me looks very tidy, my only concern is the secondary air injector, I've heard some horror stories about these.
Are they that bad if they go wrong, is there any way to check it before buying?

 

If, and when, the secondary air pump goes wrong (it will), it can be deleted. It's not needed for emissions at mot time. All you need to know is here: http://www.subaruforester.org/vbulle...olution-94534/
On mine, the fuses have been pulled, and the resulting fault codes disabled by Enginetuner in Plymouth (£336, including a handy remap with RR printout). Or you could reflash the ECU yourself, as described in the link. It's only a problem if you replace the pump or valves, they're horrendously expensive from Subaru, and unnecessary.