'Wait and see what turns up' will take ages and it will go through some really horrible looking green stages before it looks good. Make sure you have lots of shallow bits (1m and under) as most of the interesting things grow in shallow water.
In the pond, I'd go for some white waterlilies (Nyphaea alba) and possibly pond lilies as well (Nuphar lutea) if you like the look of them, common water crowfoot (Ranumculus aquatilis), water soldier (Stratiotes aloides), broad-leaved pondweed (Potomageton natans) - and any other Potomageton for that matter, spiked water milfoil (Miriophyllum spicatum)
In the edge, the pond sounds plenty big enough for some yellow flag irises (Iris pseudacorus), you also couold look at: bulrushes (Typha angustifolia), Branched bur-reed (Sparganium erectum),floating sweet grass (Glyceria fluitans), amphibious bistort (Polygonum amphibium), common reed (Phragmites phragmites) as it's so beautiful with the sun shining through it and good for wildlife, arrowhead (Sagittaria sagittifolia), water plantain (Alisma plantago-aquatica) flowering rush (Butomus umbellatus) marsh marigold (Caltha palustris)
All of the above are native so will look natural, many of them are pretty spectacular. Plant in blocks so it doesn't look bitty.
You can get chunks of plants form other people's ponds when they clear out in Autumn - but make sure they don't have any species in you don't want hitching on them.
Don't put any fish in ideally, but you could put in a few sticklebacks (they'll breed), or if you want something decorative, even though they're not native, go for some golden orfes rather than goldfish or koi. Don't add any fish until the pond's been established for a few months and there's plenty of plant cover. It will attract ducks and geese as there's an island, but don't feed them unless you want lots of ducks and a green pond with no wildlife.
I'll think of some more ideas after supper