Yea its all up to your own discretion, but i’d hold out on, the s15 front end espeically since its a HUGE investment and you dont want your nub g2 driving skills spinning a couple thousand dollar front end into a tim hortons. Gutted interior is legally acceptable, but if you cant afford 2 cars, i’d say keep the interior in until you have a beater. There’s no law that says you need an audio system. As long as you have all factory safety features installed and factory seatbelts (no 4 point harnesses) you’re pretty much good to go.
You can do with your car however you like, but just remember build and drive your car relative to your experience and make sure you dont make stupid moves like throwing a silvia front end onto a rusting coupe and driving it around without turn signals or a rebar just cause you’re to cheap to buy those. If you’re going to do a conversion, see it to the end, dont go half way and say “i’ll get that rebar later”.