At the heart of most custom keyboards is a PCB, or printed circuit board. The PCB determines how you program a board and what switch layouts it supports. The Zeal60 from ZealPC is one of the most popular PCBs for a compact custom keyboard project right now. It does not come cheap, and that's not just because of the pretty purple color. It runs powerful firmware with one of the most advanced lighting setups available in a DIY keyboard.
A keyboard's PCB is roughly analogous to the motherboard in your PC—it's where everything connects to make your keyboard work. In a high-end custom board, the PCB includes a microcontroller with user-programmable features. In the case of the Zeal60, it's an ATmega32U4 chip. Unlike many PCBs, this one is not part of a full kit (case, plate, switches, etc.). If you buy a Zeal60, that's just the start of your keyboard adventure. However, it's compatible with a wide variety of parts.
You'll need to work some magic with function layers if you build with the Zeal60. It only supports 60% layouts similar to the popular Poker 3 and HHKB2 boards. That means you don't have arrows, an F-row, or a number pad. All those actions still exist, but they're in function layers. For example, the arrows are accessed via Fn1+WASD in the default configuration. Many people prefer 60% boards because they're compact and require less hand movement.
Getting a 60% PCB for your custom build is sometimes a good way to save money compared to larger, more elaborate PCBs. However, the Zeal60 is anything but simple. This PCB includes a dedicated SMD RGB LED module for each switch. Because it's an SMD component, it's already soldered to the board. Thus, they won't complicate your build. Per-switch RGB lighting (0-255 R/G/B) is very rare in custom boards, which is why the Zeal60 costs at least $100.
The SMD LEDs shine up from under the switch, so you can't use opaque switches like traditional Cherry designs with the boring black housings. ZealPC's own custom Zealio switches are transparent, so they work perfectly. There are also numerous switches from other manufacturers that support SMD lighting by way of a light channel and transparent top. For example, the Hako switches that I'm currently using.
This board runs an open source firmware called QMK, allowing you to alter the entire keymap and change the function layers to suit your needs. You can even change the function of keys with a long-press.Both the lighting and keymap are controlled in QMK. There's no visual editor right now, so programming the board is a bit of a pain. You need to modify a batch file, then run it to upload your chosen configuration.
The Zeal60 is produced in batches that go on sale every few months. There's one on sale right now, priced at $120 direct from ZealPC.