A common problem with custom keyboards is that you often can't buy them unless you pay enough attention to catch the group buy. Of course, then you have to wait ages for manufacturing and shipping. If you won't want to wait months, one of the best entry-level custom boards is available immediately: the Input Club WhiteFox 65 percent keyboard.
Input Club ran the first few rounds of WhiteFox group buys on Massdrop, then it did another on Kickstarter with the addition of a "NightFox" variant. Now, you can just go to the IC Kono Store and buy a WhiteFox kit. The store sometimes offers fully built boards, but there aren't any in stock at the moment.
The WhiteFox is a 65 percent keyboard, so you have the alphas, a full complement of mods, arrows, and an extra column of keys along the right edge. As long as you don't need to enter a lot of numbers, I think this layout is the most efficient for most people. Most of my personal keyboards are 65 percent, and the obsession started with a WhiteFox. The original version of this board was the first one I ever built, and I still use it on and off to this day.
Input Club created this board along with designer and programmer Matteo Spinelli to offer a highly optimized typing experience. It's compact, so you can center the alphas in front of you without placing your mouse too far away. There's also a gap between the bottom row keys and the arrows, making them easier to find by touch. In the "TrueFox" layout (which is the only one currently offered), you also have an HHKB-style backspace right above the enter key. Many people find this more comfortable, but I prefer the older Aria layout with the top-row backspace. That's what my custom WhiteFox board is configured to use.
This is a fully programmable keyboard, so you can change what each key does, add new function layers, and even change the main layout to something wacky like Dvorak or Colemak. Input Club has its own custom firmware for the WhiteFox called KLL, and there's an online configurator that makes it easy to design your perfect layout. It's so much smoother than a lot of custom firmwares I've tried.
The WhiteFox kit is also an excellent deal. You can get it barebones with just the case, plate, stabilizers, and PCB for $100. You can also throw in a Type-C USB cable for a few extra bucks. There aren't many Type-C boards yet, so you might need that. If you want switches, Input Club offers Hako Clear and True as options, and you can get the WhiteFox keycap set if you want. Fully decked out, you're still under $200. You don't have to buy switches from IC, though. The kit will work with any MX-compatible switch like Zealios, Cherry Black, and many more.
If you're looking to get into building custom boards, the WhiteFox is an excellent place to start.