I think you'll be fine with the Nano (hopefully you get in on one of the holiday deals and get it for ~$65)
The Nano's 750W - only 100W less than the Bluetooth @ 850W. (which is the same heating capacity that the v1 Wifi model had.
For longer cooks and cooks that you would want to cook much larger quantities, you'll want a covered and preferably insulated vessel (the Coleman Stacker coolers have been popular for people to make their own) - they're often on sale at Walmart.
Down the road, once Anova gets all of the multicook/multistep bells and whistles worked out, maybe you'll want another device - you could consider a larger capacity one when you're ready for multiples. (heh...or not - really depends how often you would want to be cooking larger capacities)
I'd ignore the math that's been published (on how many people each model supports) - just do the ratio of wattage to volume - using the larger APC's numbers as the benchmark.