Well for the solar charger controller i was considering:
http://www.stecasolar.com/index.php?Steca_PR_10_30_en
It seems like a good choice, it's not a cheap chineese crap you can buy.
I know that the controller will keep the battery running as best as it can, but once it's depleted it will disconnect the load cicuit, and this is when arduino should kick in and switch to the generator (that's my idea). I know there might be some time between the switch (the generator needs to start and it might take a few seconds) but that's ok. Now arduino should also know when the battery is ready and switch back to it, and disengage the generator. I know i need to read a lot about charge levels of the battery, i don't know how to deal with that.
I can't switch back to the battery as soon as the charge controller re-connects the load circuit, since it will discharge very quickly, i'd need to know if the battery is charged at a certain level (let's say 75%) so that when switching to it, it will work on the battery for a significant amount of time.