CB2 Micro Is a Cheap and Easy Retro Microcomputer Kit
With this AVR-based kit, you can build your own BASIC microcomputer for less than $30 and in as little as 30 minutes.
When experimenting with retro PC hardware, there are a variety of options. You could of course source genuine hardware like an Apple, Commodore, and other more exotic devices, or you could emulate this type of functionality with a Raspberry Pi or similar hardware. Another option would be to use modern lower-processing power hardware to create a microcomputer that runs retro-style games and applications. These new apps wouldn't even necessarily have a direct relation to computers of old. As the price of components has decreased, such a system can be built quite inexpensively, €24.99 (~$28 USD) for a kit in the case of the CB2 micro.
The heart of the CB2 micro is an ATmega644P-20PU or ATmega644PA-PU chip, which connects up to a PS2 keyboard port, as well as a 9-pin connector for serial operations. Video output is via a European-style SCART connector. These three outputs will likely need some sort of adapter for use with modern hardware, but with the possible exception of the SCART connector, there’s a good chance you have something that will work lying around already. There's also an option to connect composite video from the board to your TV if that helps adapt things to your system.
The kit comes with eight programs initially on-memory, and a variety of programs that the hardware can run are available here. Besides retro-style games, there are interesting applications including a simple oscilloscope and networking applications, and it comes with a Tiny-BASIC editor when you’re ready to write your own. Build instructions are found here, which feature a soldering primer that would be good for beginners or even those that need to brush up on their techniques!