You’ll probably get a lot of different recommendations, and perhaps I’m biased (being the author of xPDO), but I’d recommend looking at
xPDO for managing a set of custom data tables in MySQL for this data. You could develop a rich API for your application, present the data in tables, easily create forms for editing them, and eventually integrate this membership data with MODx user data. As MODx moves forward, the API will be moving to one that is xPDO-based itself, so it would not only provide a great way to quickly get started with managing the custom data via MODx snippets/plugins/modules, but would also give you a jump on understanding the upcoming 0.9.7 MODx API changes...
I know the download says it’s an alpha, but it’s fairly stable, we provide support for it here in the
MODx forums, and it’s been adopted by several people that are using it within some critical production environments (including in some existing MODx sites). In fact, the xPDO web site itself is running on an alpha build of MODx 0.9.7, and thus on xPDO itself.