Mod first, finish it later.
I'm a Dick about modding and "custom", as all that means it's made by us...NOT bought. Example? You want personalization, but quickly skip over painting methods and go right for buying a panel you already have, but this one has a window and you can just unwrap it and slap it on. If you can't make a window with your side panel, pause there as you don't have the patience for painting. Prove me wrong! Anyone selling stuff, of any sort, that doesn't come stock and is bought...is "custom". No,it's aftermarket. If I hand make my fenders for a jeep and someone else slaps some on they bought on, which is actually custom? So if I make my own fan bay, reservoir, and all this stuff custom fit for me and another buys it...it's not custom is it? See...I'm a dick about it
Take off your panel and mod it! IF you fuck it up, you know of a site that sells it right? Yup. You may not want that boring window shape at all. You may envision some design or something different. Even if you like it just like the one you linked toward, then you buy it (not custom nor modded...just paid for) then you have a plain panel just sitting there and your brain can go back to...nothing artistic. Put some flashing lights in it. Call it a day...call it custom. I just passed some July mod winner (PC Perspective?) on eBay and the paint looked like a 2yo. But it had lights! Bays? Custom...bought.
With paint you have rattle can for under $100 or start buying real equipment and learning for highend sprays. That's money right? Pretty much, but both can turn out great with patience. Spend 10hrs with elbow grease, 30 minutes is spraying. That's a fender or case panel in my world. Good looking sprays end in the second a panel is slid on because the paint chips off from poor prep. It's Ferrari Red at whatever cost, but the method just sucked...no time in prep (why I pick on vendors with .com names that advertise modding here. They are all about now).
That's cut short BTW. Choose how much patience you have. If it's just a weekend, you're looking at $45 and it'll last maybe a month if you're lucky. Got time for an actual mod? Plan it out before you order parts, then plan on 12 days for just the paint (which this thread is titled). If you have actual mods, factor that in. It may be a month forward but that's expected...you want good work.
Me? I've done too many to list. They've toured with Honda, Ducati, Ford, GoodGuys, and still on tour for the Camaro...just never won a cheap mod award on some PC based mod award. It's always the blinking lights and $12 coolermaster bays that hold me back from them; and I've never tried to sell my stuff. It's the view on your blank canvas and how your view reflects your vision. If your view overtakes ability, practice more.
(or you just want to change colors. That's done just fine in a weekend on the cheap...that's a mod too)