I've done a few of these. 2 n/a 302s and 2 turbo 302-based motors. I've lived and breathed 5.0 mustangs for more than 10 years now. My suggestion is only one way to skin the cat. There are many many options available to you when building a 5.0, but this would be a damned good way to go:
With your 302 you can make 300+ rwhp with AFR 165s, 185s, or TFS heads. I would personally shy away from the 165s (which are my favorite 302 head) if you plan on stroking to 331 or 347 in the future. The AFR 185s are slightly on the large side for a 302, but with a custom cam, you can still take advantage of the extra flow. The TFS twisted Wedge heads would be an excellent selection for the 302 and would perform almost as well as AFR 165s. However, TFS TW heads have awesome porting potential and can, after being ported, flow as well any head on the market for any stroker you choose in the future.
If you go the 302 route with plans to port, my personal reccomendation for your specific application, considering you may go with a stroker later, would be AFR 185s with an Edelbrock RPM or RPM II intake manifold and a custom cam from Ed Curtis, Buddy Rawls, or Jay Allen (Ed Curtis is my personal preference). A 70mm TB, 75+mm MAF, 1.6 roller rockers, 30 lbs injectors, and 1 5/8" long-tube headers would round out the engine combo. Other bolt-ons like pulleys, and a cold-air intake are high value in the $:hp ratio.
All said and done, you're probably looking at around $3,500, but you've got higher end parts that will work very very well on a 302, and will work even better later when you go for more cubic inches. Keep in mind that if you're still running the stock 5.0 tranny, you've got a world class T5 that will break at the first thought of a power-shift, and will not last long behind a 300 rwhp combo either way.
With the combo laid out above your peak number, depending on what you ask for in a cam, could be well over 300 rwhp. I made 320rwhp with a 302 that had a ported cobra intake, AFR 165 heads, and a custom cam (very mild street cam) from ed curtis. I think you'd make better peak power than I did with the combo I listed, but average power might not be.
With the same parts on a 347, I'd be looking for around 400 at the wheels. Only parts to change out would be the cam and the headers.
Good luck with whichever route you decide to go.
Chris