Any of the usb floppy drives that claim Mac support should work fine. And at $20-$40 there's not too much risk involved (in case you get completely ripped off by some fake company). I'd buy retail (I hate Amazon, but that's just me). Walmart seems to have a decent collection of drives.
I'd stay away from "USB 2.0" drives for this. You're paying for more "speed" but the speed is going to be limited by the floppy anyway. You'd be paying extra for no good reason. USB 1.0 is enough for this.
To destroy them afterwards ... you can use the Mac to quickly erase them (just deleting the file system info), or to write zeros to every block (in case you're worried about someone recovering the data). Or you could use a degausser (or strong magnet). Or pry off the floppy gate with a screwdriver and take some shears to cutting the floppy. Hmm, looking at it ... just take something sharp and slide open the gate and cut the disk. Once it's damaged like that nothing normal will ever read it again.
The real question is what kind of brew to consume while wading through 50 disks worth of files. It's going to take a while to review 
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