i would put my money on head gasket, passenger side rear of the motor. firstly tear down your motor and see what has happened, if the pistons rods and crank are all fine, i'd just re-use them.

if all of these things are fine...
chrome/moly rings are just a different type of metal, and a lot harder. they take a lot longer to bed in than the standard cast rings (up to 3000km's) and you do run a small risk that they wont bed in. having said that they are a good thing and cost less than a set of genuine rings from nissan roughly $100 - $120.

no need to balance your crankshaft if you are using all of the same bottom end components as it is done from the nissan factory.

i believe forged pistons are only needed if you are going for some really big power and want to tune your car really close to its detonating threshold. if your engine is pinging it will last 10 seconds longer if the pistons are aftermarket forged items. the factory CA turbo pistons are a good thing.

don't o-ring the head.

there are a few fairly priced clutches on the market. adelaide extreme, daikin exedy, just have a look around and speak to them.

i rebuilt my CA18t with fresh bearings and molly rings. the rest is standard. at the drags i was running 16psi (T28bb) and revving to 7500-8000. my pistons are fine so slight boost increase will be more than ok.

Just get it rebuilt to standard. USE A GENUINE NISSAN HEAD GASKET. $90, can't be beaten! buy a complete gasket kit for the rest and throw the head gasket on ebay or the bin.

or you could buy that other fella's CA18 package for a 1200, its very fairly priced for the motor alone!

good luck