Hopefully this doesn't come across as a port beg. I've got a hankering to try one of these games out, but I don't have a switch yet and probably won't get one until 2018 sometime. Or until Mario breaks me but anyway.
Consoles I have access to easily-
3DS
iPad (new one)
a decent PC
Xbox One
PS4 Pro
Wii U
What should I jump in on? There are so many now and different versions of each. I'm in Australia if that makes a difference. What's a good starting off point for a beginner? The PS4 version seems to be my preference, but what is missing from the 'complete' edition on Switch? Should I just wait?
The PS4 version of Disgaea 5 plus it's Season Pass basically is Disgaea 5 Complete. Outside of a minor pre-order exclusive from Gamestop which was for a Bunny costume for one of the main characters. With that the PS4 + all DLC is essentially content comparable with the Switch release.
I may suggest picking up Disgaea 1 PC on Steam since you said you had a gaming PC, as that is the introduction to the Netherverse, however it is the weakest in the series mechanically. Each game just builds upon the previous installment truncating redundant mechanics and adding entirely new ones, so with that said Disgaea 5 is the most refined from a gameplay standpoint compared to the previous titles, and I won't be surprised that Disgaea 6 refines the formula even more from Disgaea 5.
Storyline wise there is no direct connections for each game. Meaning you are not required to play each game in order to understand everything, however there IS various cameos and references to other games, as well as some returning characters from previous games that show up as bonus recruits, so jumping into D5 the origin of these characters may be lost on you and the references they make in their text dialog might not make sense, but not so much to take you out of the game.
If you plan to eventually get a Switch, I say if you absolutely cannot wait, get the PS4 version, or get Disgaea 1 and Disgaea 2 PC during the Steam Summer sale (Grab Phantom Brave too I suppose)
Play those games get a feel for the mechanics see how the gradual improvements are from D1 to D2. Then 2018 should roll around by then Disgaea 5 Complete will still be available to pick up when you get a Switch then, or likely released on Steam, which at that point you will be somewhat acquainted with Disgaea's level of chaos, only to just take everything up to 11 in Disgaea 5.