I'll put it this way. All the information you're getting is good information.
What you have to do is determine exactly where you want to go on this build. You said 300HP. That can be done on a 318, but you have to determine if it's worth the cost, vehicle downtime, work etc.
The heads and pistons are not the last things you do when building an engine. Actually, you work from the inside out. The first thing you do is make sure you have a solid rotating assembly: crank, rods, pistons etc.
You can do one of 2 things with the 318 in your car.
1) Do the basics: cam/intake/carb/ignition/exhaust/valve springs. These are the easiest, most common, and "cheapest" things to do without taking your engine all apart. These things will add power, they are modifications that, except for the cam, are almost impossible to make mistakes on, and they'll help you begin learning what this is all about. When you are done with these mods, you can get in your car and drive it with almost no worries. The upside: it's cheap, easy, and you can get in the car and notice it's faster. The downside: you will be stuck with this power level until you decide to go to the next step below, which, you may find you're content with just doing step1. If you do decide to go to step 2, you'll have to do this all over again....
2) Do a complete build: This means heads, pistons, bolts/hardware, machine work etc and all the other "expensive" things. This is the right way to build real power and get the full potential out of the parts you choose. However, at this point, there is no point in messing with the 318 unless you decide to stroke it or do a 360 swap. This is the point where you'll have to research what exactly you plan on doing for the engine. If you decide on going this route, everyone here can help you pick parts and give pointers of where to go with the build. The upside: you'll have exactly what you want for a motor AND if you build a solid foundation.... heads, pistons, crank, rods...you'll be able to add more down the road if you want to experiment with different cams etc. The downside: this puts ALOT of downtime on your sole driver. It's also alot more expensive. You may get to this point, as a daily driving car, and realize it's too much car to drive everyday.
So it just depends on what you really want. I think doing the basics will make you more than happy for now. Like I said before, I plan on using the same 360 heads when I do my build. I'm not really worried, I think I can get my parts list right and with some needed machine work I think it will be fine.