$2300 is pretty cheap for a well built A15 I spent close to $5k on my old JTS A14 and made just over 150hp was very reliable and at the time was suprising quite a few high tech turbo cars!!! Remeber when the WRX first came out the old A14 would stick with them to around 160kmh !!not bad for a little pushrod engine. Yes Ca18dets go better but a lot more hassle and expense. Good luck getting one in and running sweet for under $6k engineered.
Cheers Andrew
PS: I still miss the sound of my old A series had the nicest sound at around 7500 If I where to build another 1200 I would probably gor for another Aseries and thats after owning a 230rwkw ca18 coupe!!!