The main point to remember with AIO mods is that because the battery is built in if it runs out then you can't use the mod until it's recharged, and, if the battery fail completely then the mod will have to be thrown away. If you get a mod with replaceable batteries then you can carry a spare (In a protective case) and a faulty battery can simply be disposed of and a new one fitted. The initial outlay may be more but the long term savings are real...
A fair bit has changed since this thread was started back in November of 2013. Back then a 40w mod was considered high power and there were only one or two that went to 100w or so, mechanicals were much more popular and most mods were tube types. Batteries were also far different as were attitudes to Sub Ohm vaping.
While it's still important to learn about battery safety and to know your batteries limits the situation is far more settled than it was then. I can now safely reccomend just one or two types of battery in the knowledge that they'll cover pretty much everything you'll ever need. for "Regular" vaping - So I'd say Get Samsung 25R's or LG HE2's or HE4's. They should cover most of the regulated mods on sale now and for the forseeable future.
Go and have a read of the guides in the battery section -
https://www.planetofthevapes.co.uk/guides/advanced-vaping/battery-guides - They should tell you all you need to know.