I would go into modding if I could... Being 15 years old, I don't really know where to start or how to get the tools I would need, not to mention, money is not unlimited.
Building a proper case from scratch requires more skills, more tools and more money than modding an existing one. The more you go into modding, the more cases you disect, the more you learn about the notion of case design, the how and the whys. You can't really skip it. Well, you can, but then you will learn these things the very hard way, by your own mistakes on your own expense.
That modding would need some plastic sheets, a drill and some tapping and a rotary cutter. For starters, for ~$120 you can get a digital caliper, a hammer, a pair of plyers, some scredrivers, a set of hex keys, a tap and a Dremel tool with bits to do the drilling an the cutting. Add a little more to get a vice and maybe a work table. There, you have a very basic workshop. You can also go for used stuff. Not as good as the new, but perfect for learning.
You will need to learn how to measure, cut, drill, tap, and bend. You can't do any of these properly without the right tools. On more advanced builds, you will need to mill, lathe or weld. You can learn to use them sooner or later, but in order to do nice builds, you need to do it. As you go on, you will start to understand each basic tool. Then you will be able to buy better, more specialized tools to update and improve your workshop.