I haven’t used the Nissin myself http://www.digitalrev.com/product/nissin-di866-mark-ii-flash/MTE4MTI_A
but DK used one and said it wasn’t a bad flash (from memory), the review above sort of gives it a so so rating so I would be inclined to go for the best if you want a reliable unit. (Metz)
You can go to the Metz home page http://www.metz.de/en/lighting.html
and browse through or give your camera model and make to the selection guide and have a look at the suggested model/s.
As far as using a slave trigger goes you will probably find that there are a couple of gotchas involved, one is preflash which can on non-compliant slaves trigger the main flash on the preflash and the main flash is recharging at the time of the actual flash/shutter sync.
When WL is working properly the onboard flash is only supposed to contribute about 25% of the total flash (or so I’ve been informed) so it’s contribution should be almost unnoticeable if WL is working properly, if you are getting exposure from the popup there is a gremlin in the works, bad comms or non compliant WL flash or slave etc.