Taxes to rebuild

Welcome to Stangnet. If you want to run nitrous, or any power adder for that matter, it's a very good idea to run forged pistons. They can handle a little bit of detonation. If you run a 306 with AFR's, a good intake and cam and add nitrous you could easily see 450+rwhp. Even a naturally aspirated AFR setup can get you around 350rwhp with the right components. For street use, flat tops or dish pistons are usually the way to go. Domed pistons are more for race cars with extremely high compression and usually require that you use a high octane race fuel.
The E303 is old technology. I would say if it's feasable, get a custom cam for about 300. Otherwise, one of the Trick Flow cams would work well and are priced even better than a Ford alphabet cam.

H beam rods are beefier than I beams. If you want a good price on a good forged rotating assembly, check out http://adperformance.com/rotating_ford_302.html. They charge 1300 for new 306 rotating assembly with Scat forged crank and h beam rods and Probe forged pistons, or if you want to spend 50 bucks extra you can buy a 331/347 stroker kit. It's a little overkill for a stock block, but it's good insurance in my opinion.

I'm tired and this post probably didn't make much sense, but hey it will get you started.

-Dave