PowerBook 100-series machines are awesome. One thing you've got to watch for, however, is the plastics. Also the hinges. The plastic around the hinges likes to break, and when it breaks, it's pretty difficult to execute an aesthetically pleasing repair, as the screw mounts are usually ripped off. The only recourse, typically, is to use some through-the-whole-assembly bolts or the like. Not pretty, but it often works.
The PowerBook 100 is a bad choice unless you can find some working batteries. I have personally not seen any for sale, at least not with an acceptable asking price. These are lead-acid, btw.
Others aren't too bad. Obviously, the ones the a 'c' at the end are colour. The 170 and 180 models have active-matrix screens. The rest are passive. Most models provide a full compliment of ports, except the PowerBook 150 (which doesn't even have ADB). In fact, the 150's only saving grace is the fact that it uses an IDE hard drive. Unless, of course, you also value a 640x480 screen (as opposed to the 640x400 of the other models).
The PowerBook 190 is a 5300 with a 68LC040 instead of the little 603e; almost everything that applies to the 5300 holds true for the 190, except the fact that the 190 only has passive matrix screens (one model black-and-white, the other colour). Good points about this model include the PC Card slots, the IDE hard drive, IR, and the removable floppy. On the down side is the fact that the power adapter is specific to the 190/5300-series models, so there may be a problem sourcing one.
As for SCSI Disk Mode, you'll either need a special cable, or a little docking dealie (can't find a brand on mine, on one side it says, "HDI SCSI/DOCK Adapter," on the other it has a switch to turn SCSI Disk Mode on or off). eBay would probably be your best bet for finding either one.