Some of the issues are that you're new. We just don't know you too well so it's hard to give exact help. Post some pictures of the car and of the 302 you bought. Like the saying goes, a photo is worth a thousand words. Again, I'm not trying to push you into what I did. It was right for me. Some of it comes down as to what's available and how much you want to spend. One of the problems with forums, you may just need a tune up and before you now it, you'll be building a 347 with big heads and a big mechanical roller cam. Not what you wanted or needed in the first place. Don't laugh stuff like this happens.
Mustang 5.0's were and are popular swaps. But the last Mustang roller cam 5.0 sold was 1995. That's 22 years ago. These were driven hard. Back in 1995 you could find a lot of 87 to 95 motors cheap because a lot of them were crashed. The last roller cam 5.0 pushrod motor was 2001 in Explorers. Not quite the same motor but still a very good motor. They used much better heads, milder camshafts but the big advantage is they usually weren't driven as hard, usually soccer moms drove them and they are a lot newer so a better chance of finding a lower mileage motor that's still usable without rebuilding. Even with the milder cam but better heads then the Mustang 5.0, the Explorer motor will probably make more power with the same exhaust system of the Mustang. Basically what I'm saying is if you're just looking to install a complete running motor that you don't want to rebuild or modify too much in stock form cheaply this may work for you. I would estimate my motor is about 250 ish HP, maybe slightly more. It may not sound exciting but it runs real well. These cars are light, so you don't need a lot of power.
There's a ton of good used parts and a ton of cheap new parts out there for these motors. Stock heads can be made to work very well and there's nothing wrong with fixing them up other then for the expense of doing this. You could end up spending more then just buying a new set of aluminum heads. On ebay a vendor, Skipwhite sells offshore castings but does all the machining in house with USA made parts from Comp Cams and Crane. for $699 a pair I'm not looking to buy a pair but I think you could very easily spend more on stock heads and make less power. From what I've been reading on late model forums, these are pretty good heads both from quality and the power they make. Personally I'd prefer to buy USA made heads. But from the cost standpoint of either rebuilding stock heads or these. I think I would buy the Skipwhite heads. It's not a recommendation to you just what I think I would do in my case.
Tell us more about your car and exactly what your goals are. Don't be in a rush or let money burn a hole in your pocket. Post pictures, we love them!
I'm not a complete idiot, pieces are missing.