Around here, the ej20t, ej22, and ej25 are more common. Many guys seem to have the ej22 or ej25 in NA form. The ej22 seems to have a solid reputation for being reliable and last many miles/km (not to mention they are CHEAP and easy to work on). I've read where people considered the ej18 heads on the bigger bottom end, but since it's easier to just get an ej25 block and hybridize with ej22 heads/top end, that's more common. Also, I think compression ratio is something to consider, as once it's starts going past 10:1, it's not always practical to run, especially if you can't really alter timing advance (slightly thicker head gaskets are a solution, especially on NA setups). I'd have to think an ej22 bottom end with those heads would work OK, though I know nothing of the port sizes. If willing to experiment, could always try an ej25 bottom end with you ej18 top end as I'll bet it'd have SUPER responsive throttle response, but might lack top end power. Velocity through the heads would be crazy. Around me, I can get ej22 or ej25 engines for $155 all day long with a $55 core which isn't worth giving a good engine as a core for that price, so $210 is more realistic on low-side of the curve, whereas $300-500 seems to be norm. The swap is easy and straight forward.
Since cars are harder to keep on the road down under with your regulations, you might be better off getting a salvage car at auction for $300-500 that runs. Take everything you want from it, then sell the rest for parts. You can easily come out ahead with profit after it's all said and done if you don't mind wrenching. I spend time in the Saab forums where a mate from your neck of the woods has picked up at least 2 NG900's from auction just as parts cars and came out ahead on both cars. Both cars still looked road worthy too, so I'm guessing Subarus should be easy to locate. Personally, I think the ej25 block with the ej22 heads sounds like the nice way to go. I'm not sure on the bhp on a stock setup as no one ever seems to mention it, but if an ej25 SOHC is 165hp, and adding the ej22 heads increases hp (maybe 10-20 at most), that'd be an easy 60-80 hp over what you have now with a bunch more torque and probably very similar gas mileage with nothing but some wrenching and a weekend. If I keep my Legacy for awhile, I'll probably do the bottom end swap for the extra tq and 30+ hp over what is now. Figure it'll increase the value and novelty slightly to a dieharder and make for a little more fun.
Edited by Bushwick, 01 November 2013 - 12:28 PM.