The MAME 2003 Reference Set is a , not a museum. It gives up some accuracy to achieve what no other MAME version can: playable, full-speed arcade emulation on a $35 computer . If your goal is to build a bartop arcade or a portable retro machine, hunt down this specific set (look for the Reddit "MAME 2003 Reference Set - Full Non-Merged" thread). If your goal is archival purity, look elsewhere.
Never use ROMs from a newer MAME set (like 0.250) with a 2003 emulator. They often won't boot because the internal file structures changed.
If you need a specific section expanded (e.g., how to convert a newer ROM set down to 0.78, or how to configure RetroArch for CHD paths), let me know.