A third battery, and spare charger (not necessarily the wall wart PS, as long as you have a PC), are great to have.
As others mentioned, a standard passthrough, with a 2A USB wall power supply and/or 2A car cig lighter adapter, is also excellent.
If you are content with 808 equipment and cartomizers, there really is no particular need to go to 510 equipment. I used 808 for half a year, a Riva 510 with my 808 stuff as backup for several months, have been using a mod the last five months (with an 808 battery/charger backup). I've never been an atomizer person, just not for me.
That said, a Riva 510 Lite kit, two batteries/charger/couple atomizers/carts runs just over $30, is an entirely viable alternative. In my case I used boge 2 ohm 510 connector cartomizers with the Riva -- decent warmth/power.
I'll credit the Riva 510 with good run time and low price point. On the downside, on the Riva batteries it got to be a real PITA for me to have to keep prying up the center conductor (some cartos would push it down slightly, and you had to pry it up to get good charger contact or when using another carto). I never had that problem with 808 batteries, or with my mod.
Whatever way you choose to go, just make sure you have some kind of backup.