Unless you really need wide spacing for slapping, try to find a 6er with narrower spacing and a narrower nut-width, for your first one. It's a big jump from 4- or 5-strings to 6 as far as wrapping your hand around the neck. As your technique adjusts to accommodate a wider neck, you can start playing with wider spacing. Most 6-strings have 18mm spacing or less. You might also look for a 34"-scale for your first one for the same reason.
Balance becomes very important, the more strings you add, because of the extra weight at the headstock. Look for an instrument with lightweight tuning machines, and/or with a longer upper strap horn (extending to the 12th fret or so).
I'd also recommend not starting out with a very expensive one, until you get used to it (unless you just happen to find a great deal). A sixer is not for everyone and it's common for people to get one, try it out for a few months, discover this, and sell it at a loss. On the other hand, you don't want to get one that's cheap, because a good setup job becomes increasingly important on a big neck with a lot of tension from 6 strings. Many sixers have 2 truss rods and reinforcement with carbon-fiber rods or wood laminates. Start with the action low, until you get used to the bigger reaches, and then raise it for tone gradually. Don't strain; if it starts to hurt, stop, stretch, and wait awhile before you start playing again.
Also, don't get turned off by the tone on the C string, if it seems tinny at first. It could be your EQ settings and your amp, as well. Not all amps that sound great for 4- or 5-strings sound good with sixers, and the high C string tends not to sound very "bassy" in the first place. Adjust your settings until you find a tone that works well across the range of the instrument. A quality preamp becomes more important with an extended-range instrument.
As far as specific brands or models, there are so many to choose from... It depends a lot on your preferences and your budget. What kind of neck shape do you like? What kind of tone (what amp are you using)? How much do you want to spend? The more info we have, the easier it will be for us to recommend something for you. That said, the best way to find "your bass" is to play as many as you can and just see what feels good to you.
Hope this helps.