A possible reason for playing 5 string instead of 4 is that you can play in the middle of the fretboard instead of being stuck near the nut. For example, play G on the 8th fret on the B string instead of the 3rd string on the E string. This would be an advantage for someone with small hands. I have small-ish hands and when I play 5 string, I rarely play below 5th fret unless it's one of those handful (literally 5) of notes on the low end of the B string. Not only is the space between notes smaller in the middle range, but the angle of wrist and elbow are much more comfortable in the range between about 5th fret and about 12th fret (depending on instrument, player, etc.). Of course, this is a vote for a full scale (~34" scale) instrument rather than a mid- or short- scale instrument. I think a short scale 5 string would have a harder time producing good tone on lower notes, but I'm sure there are exceptions.
I would pay a lot of attention to string spacing, and not get a bass that is too wide.
As for playing left handed, the only disadvantage that I can think of is that there are fewer options for left handed instruments. But you just need a bass that works for you, not a collection. When you first pick up the instrument, your fingers will feel like wet noodles regardless of if it's right handed or left handed. Then you'll practice and be able to express yourself with whichever you choose.