If you're struggling to get an SLC based device, don't fret over MLC. Reason being is SLC is soon to completely go extinct. MLC isn't too bad these days, with new garbage collection and wear-leveling techniques.
If you're heavily configuring this Linux build, just try to cache as much as you can, to offload the work from the USB drive, and as long as it's not only small but "light" in workload, most reputable USB drives will work. Formerly working for a "name brand", I know said name brand, standard issue MLC-USBs are used in Dell and HP servers to load VMWare ESX with a lifespan expectancy of no less than 5yrs.
Again, keeping with MLC, don't completely fill the USB drive; e.g., if your build is 4GB, go with at least 8, if the build is 8, go with 16, etc.
Whatever you do, DO NOT go with an SD card. The IO for an SD card is not friendly with OS thrashing and you'll burn through them pretty quick or find that your data is regularly being corrupted.
The new "MLC" to avoid is "TLC". MLC=2 bits a cell, TLC= 3.