Start Cheap! I'm talking about the POS cases that no one recommends in ANY reviews. My first modded case cost $15 from Fry's Electronics and was just a steel frame with panels...Ahh 1995, how I miss thee.
If you screw up a $15 case you won't lose your shit with any screw up and won't feel bad about trying to fix or work around it. With a $70+ case you let your emotions ruin the entire trip and get hasty. I've seen too many get a good case, attempt to mod it, screw up, then give up and just add lights or whatever...settle. That's not the way to enter the modding world. If you can turn a cheap case into something you really like and put effort into, then you are a modder. Will it win awards? Who cares!? It's a start. It's bootcamp for modding and eventually you'll work up to more advanced stuff. For now, you need to work on your abilities.
The last case I modded top to bottom cost $400 and was a few years ago (I prefer "transform" here). These are the cases you do NOT want to mod right away and will catch huge grief if you just tinker with them learning your craft. I swear to you that you want to practice on a $15 case and not a $400 case.
The investment is not just the case but more the tools as they'll travel with you further than that first case. Center punch, good bits, good ability to use a drill correctly, looking up paint prep and not just covering beige with your rattlecan color of choice, a rivet gun...that's all 10x the cost of your host case.
What you want to start off with is a blank white canvas to work with. White Canvas are about the same cost and you're not afraid to toss them in the trash and grab another for another shot with what you learned from the previous attempt.
Above all, be tasteful and useful in all mods. If it's for show, say so and be honest or old modders will be brutally truthful with your art. If you put 8 fans in a case I'll come looking for ya
Like a carrying handle on a Tower; That thing will bang your knees so many times going to LAN's that you'll just carry the damned thing in your arms and the handle is all show. Handles belong on mATX cubes. I'm a silly bastard that drug around a Cheiftech/SOHO full tower around with handles back in the day...fuck that handle.
There's a whole world of modding here and you should feel open to criticism and assistance. People paint cars and get pissed about one little flaw...this is a computer. I know this feeling to this day and know where you're coming from. You're starting off, learn the basics. Peers are more impressed watching you learn and work things out than they are with bolt on shit. Modding is doing your own work.
I'll back and help ya bud. Slowly, but surely!