When you'rre talking about a 302W based engine vs. a 351W based engine, you'll find that the cost differance is rather small. Once you "step up" to a Lima or FE motor, costs are significantly higher. The advantage to the FE motor is that it's historic. It looks good, and it looks right. Same thing with the magical 427 cubes, it just feels right. If that's what you have to have, then build it right to begin with.
The advantage of the 460 motor is... well... ummm... hang on, it will come to me.
This is my second Cobra build. The first one was not exactly built on a budget, but I did decide to be "reasonable" and used a moderate build 302 with a carb. It was a fun car, made good power, and was reasonable to build. Not too expenive, not too complicated.
But, I quickly realized that it wasn't enough. Within a few months, I was planning power upgrades. It soon got to the point where it was either tear it apart and rebuild it. Or sell it and start over. Fortunatly, I decided to sell and build another one. I ended up with a much better car the second time around.
The point is, don't be reasonable. This is your dream car. Build what you want the first time around. If you want a 600hp car, then build one. If it has to have 427 cubes, or 550hp, or an FE motor, then build it. Don't scimp - you won't be happy. Dream big, build big.