The 510 system will definitely give you the most options. I've become a fan of 808's though, so that's where I choose to stay. I've noticed i rarely have to worry about my cartos being out of stock, even during Chinese New Year.
If you want to stick with automatics, the Echo can't be beat. It's an automatic fat battery from 650-1300 mAh with a 3 ml cartomizer. I only use the 650's and they easily last me an entire day at work. However, your 901 atties will not work on this battery without doing some fiddling to an 808-901 adapter.
For the best eGo-style 808's, I'd say go with the X2 line at Smokeless Image. The X2's are pass-through charging batteries, so you can vape while recharging the battery. They're not cheap, but they are solidly made and take a ton of abuse. S.I. also has a VV battery now, but there are better options for that.
NHaler has both an iGo and an Echo VV batteries known as the Revolver. The Echo version isn't a "true" Echo, it is merely flat-topped with one set of threads. The iGo version has 2 sets like an eGo. For $28.00 you get a battery and charger, as well as a CE4+ with 3 heads. NHaler also has the iTaste mod, if you want to go that route.
MadVapes is your best deal on mods, with both the Gripper and the Roughstack in 808 at reasonable prices. I've heard very good things about the Gripper.
NHaler, S.I., Smoktek and GotVapes are your best sources for 808 clearomizers. S.I. doesn't offer anything in varying ohms, with everything being in the 2.8-3.0 range. Hopefully that will change as time goes on now that they have a VV battery.
Just remember that 901 atties will not fire on a "sealed" auto battery without some modification. However, they will work on manual batteries with no issue.
Good luck!