My first project on returning to this hobby was to built an RC2014 system.
The RC2014 is a modular Z80 based computer with boards and kits available from its creator, as well as modules from third parties.
Details of the RC2014 system can be found here.
My set up runs CP/M, but is mainly used as a software development platform for the Small Computer Monitor and associated software.
The Small Computer Monitor supports all of the standard RC2014 kits, with either of the official serial modules (6850 ACIA or Z80 SIO/2) or SIO/2 modules following Grant Searle’s original register address order. The program detects which module is available.
For a broad range of information about the RC2014 system, check out Marten’s RC2014 Infobase