The best “kit” at the moment is probably a solo from amazon or the like. If you want to build a multicopter yourself (more fun), there are lots of options. You need to start by deciding some performance parameters (lifting capacity, size, and endurance) then start designing your system and selecting components. A lot of people use a website called ecalc to help.
Regarding arduino, it depends on what you are trying to do.
The ArduPilot codebase isn’t written for or in arduino anymore - the name represents the origin of the project, not where it has advanced to.
Interfacing an external micro controller, such as an arduino board, is best done using mavlink. MAVLink Step by Step
I hope this helps.