So mini-towers are not expandable, correct?
No, the is a broad generalization. Each maker has their own idea of what "Mini-Tower" means. With HP it means at least to bays available. With Dell it mean no extra bays. And vi wail vary by model even with the acme Manufacturer.
You may have to think external storage.
If it were myself, I would have to chose between getting the PC I really want or getting on e with more expansion.
I would have to compromise and geta the best PC and then swap out a larger case to hold the extra drives. Might even have to upgrade the PSU. As an alternative, I might invest in a network server to host drives. With gigabit Ethernet it is very nice way to add storage and keep stuff off the desktop.
Also, consider eSATA as a sway to do external storage.
http://en.wikipedia.org/wiki/ESATApeSATA gives is better than USB.
In computing, eSATAp (also known as Power over eSATA, Power eSATA, eSATA/USB Combo, eSATA USB Hybrid Port (EUHP)) is a combination connection for external storage devices. An eSATA or USB device can be plugged into an eSATAp port.
...
eSATAp throughput is necessarily the same as SATA, and USB throughput is that of the USB version supported by the port (typically USB 3.0 or 2.0). eSATAp ports ... can run at a theoretical maximum of 6 Gbit/s (bits per sec) and are backwards compatible with devices such as eSATA 3