Another vote for used and shopping CL, yard sales and the like. But shop carefully!
I started not much older than you with a list of what I had and didn't for sockets in my wallet. I gradually filled in the blanks I needed, and as many have said, you don't need all of them. You can do the same with other tools and with today's tech you could keep that list in your phone...
A comment on the Craftsman you have at work: If more than a couple of years old, those are probably all US made and good quality. In the past 2 years Sears has been outsourcing to China/Asia, and still charge the same $$ for no better than HF... If it doesn't say "USA" on it from Sears, just walk away.
You are looking to go into diesel mechanical work so you need to be looking for big stuff unless you already have it. Again shop it around. You can get screaming deals if you are patient as there is not a lot of demand in that space. Not like there is with automotive tools where you have both pros and hobbyists as potential competitors.
Key question, and several have mentioned the topic: What exactly do you think you need to do your job? Make a list, Prioritize it for "need sooner, not later", then adjust you're attitude when you find it. Something you need now, be willing to bay a bit more. low on the list? It better be a screaming deal.
In the past year or year and a half I've collected full sets of 3/4 and I'm working on 1" drive sockets, breakers and ratchets are compete in those sizes, and all for a pittance. I'm not even a pro mech. I just like having the right tools when the need comes up, and with me, you can never say never...
No, not much matches brand-wise, but if i need a size, and the extra beef, in the larger sizes, Its' there, and in quality brands... (Mostly Proto and Snap-on with a smattering of other truck or industrial brands)