The best is to buy a GOOD external 5.25" case and the burner that you prefer. There are some rare cases of this size that also have FW800, eSATA, but those interfaces should not be worth it, given that opticals are so slow (I had one such FW800 case that stopped working).
Any drive should work. What happens it that for many you need Windows if you have to update the firmware.
If you want to spend as little as possible, anything that is not Super Multi (DVD-RAM too) with LightScribe is not worth it. This used to be LG's turf, but now several other brands offer drives with such specs.
Going up (double), one could perhaps take a look at the Super Multi, LightScribe, BD-ROM, HD DVD ROM drive from LG (at least to watch hi def movies in Windows).
Doubling up again would buy you an LG Super Multi Blu with LightScribe that can burn BD-RE.
But I wouldn't buy a ready-made external that doesn't state it's Mac-compatible. The case is the issue, not the drive. There are many flakey cases.