Thought I'd see if there were any likeminded individuals around.
This is three of my small collection that I have with me at work. The 4G Classic (left) is modded with 64gb, the Mini is 128gb, and the 7G Classic is currently in need of storage and battery. My plan for that one is to give it 512gb and as big a battery as I can squeeze in there.
The Mini is one of the easier, cheaper ones to mod, because the hard drive uses a Compact Flash connector. So you can either replace the drive with a standard CF card, or you can buy a CF>SD adapter and get more bang for your buck.
The 1st gens can only go to 128gb, but the 2nd gens can take 256gb.
However, they are slightly more awkward to take apart than a regular classic iPod.