Plans for a great adventure!
More research will improve your chances of success. You might consider "flipping" your trip and going north-south, giving you good control over the timing in North America. You could fly into Vancouver from the UK and travel out from there. The most expensive and heaviest gear will be needed for the North and can be shed as you go south. You'll start in safer territory where you speak the language and you can see if you like the on-the-road lifestyle.
Self defense is a tricky subject. I agree that bringing a hatchet to a gunfight is not a good thing. Being robbed is a real possibility and they will have the element of surprise, so most self-defense scenarios fall apart, regardless of how well armed you may be. I too carry a "sacrificial" wallet with cash, credit card and traveler's checks stashed elsewhere. You may want to get your traveler's checks in US dollars– research that. There's no way I would carry all the money for the journey at one time– get re-supplied along the way.
I agree that dealing with the local police is a concern. Keep your appearance up and don't carry drugs– even a short stay in a third world prison would be more than you ever bargained for. Your may be in some places where bribery is a way of life.
If you are in Central/South America, your will find machetes everywhere, which is about as good as you'll get for light cheap wood chopping (and self defense). In North America I recommend a good quality folding knife with the blade 3.5" or shorter. One of the Victorinox Trekker knives would make a good tool for your journey. If you want a knife with no tools, the Benchmade Griptilian is an excellent choice. A folding saw is a better bet for wood cutting and weight. Again, the local police may not care for a backpacking young man with 2' of machete sticking out of his pack– backwoods stuff only. Use it and give it to a local when you head back to a town/city.
Do get fluent in Spanish– that will probably work into your university plans easily. Prepare for the shock that your university Castilian Spanish may be very different from the local dialect. Keep smiling.
For backwoods travel in North America, the UL gear lists you see here will serve you well. For Central/South America, you will be dealing with bugs, heat and rain. I don't have good referrals for resources there, but I'll bet there are some good travel journals where you can gain the experience of others who have gone before you. Google "backpacking Central America" and you'll get 185,000 hits :)
Do PM me when you go through Seattle!