Sounds like your after NA hp so i guess my opinion won't count as much.There is always going to be a car/engine with more hp/torque than what one has.And even after you achieve targeted power, you will want more.and thats where turbo will seem as the most value for money.So maybe ca18det would be your best option in the long run.
That said, i would advise you to A12 it as it originally came out with it and live it NA simply for the fun/enjoyment factor.Or any A series engine will do.Spend all the costs of installing the sr20 on modifying/upgrading the a12engine and its drive terrain,56a 5speed,nismo lsd etc.. will end up costing more but will also be more rare n still really old school.
You keep the old school factor and in the future i doubt there will be many a12 NA powered dattos with the current trend...
i.e running such an a12 setup,great idle and sounds good.
http://www.youtube.com/watch?v=mEx5yrDCAno&feature=relatedand the worth of your engine based on opinions is really based on opinions and everyone has a different one.As some have said and tried to sell worked a series engines on this forum, some see the value and others don't.some will question why pay 1500 for a modified a12gx head when i can buy a ca18det etc etc.... and others will buy the gx head no questions asked.
What are your plans for the car?
Cheers