For the weight limit of the frame, I would look for 15C lipo locally... I'm sure it's there or available at reasonable costs from neighboring areas.
30 mph on a 16" right?
30 * 336 = 10080 / 16" = 630 RPM...
I would consider RC to be about your only choice here...
Normal geared hub motors in a 26" wheel at 36v (10S LiPo/12S liFePo4) will average about 18mph max to 15mph at battery cut out.
The idea of overvolting the heck out of a geared (for weight savings) hub and running it at 630 RPM (about 2x the design limit) will cause serious overheating...
Your better option would be even to look at a single stage RC drive system - or somthing similar, mount the motor behind the seatpost or beneath that same area - and direct drive it to a gear on the left side of your rear wheel, for lower power stick with #25 chain as it's lighter and still good for about 2-3kw.
Assuming you got an RC motor like a Tower Pro 5330 9T 235kv and approx 80% efficient - so again,
235 * .80 = 188 loaded kV (RPM per volt input)
Lets look at smaller packs, either behind your seat tube or mounted where the water bottle bosses are in the fore part of the main folding tube...
For arguments sake, lets say... 6S packs
Again we work with Nominal voltage figures so 6 * 3.7 = 22.2v nominal
22.2 * 188 = 4173.6 RPM - this is what your motor would produce at the drive shaft at nominal (average) 6S pack voltage...
If your going for 30.. we figured it at 630 RPM on 16" right?
4173 / 630 = 6.62 : 1 reduction required between rear wheel and motor to acheive 30mph sustainable on 6S packs.
Chargers for 6S packs are cheaper too... throw 4 packs together (15-20C) @ 5AH each for 20AH then set your current limiting around 1200w or:
1200 / 22.2 = 54A
Now you will need more AH for the range you are looking for, that setup would likely get you 25-30mi without pedaling maybe more even... Add a standard rack to front or rear and double up with another 20AH... if you can't get 50mi range out of 40AH of battery, somthing is wrong
If you are dead set on getting a hub motor, go for somthing geared but designed for Higher RPM of your 16" wheel - I can't think of any appropriate hub motors for that size off top of head but perhaps a PM to cell_man could help there - I know he has direct access in China and maybe could get you setup with some 16" mag style hub motors I've seen... not sure they would even fit.
Nice part about RC or BLDC motor design direct drive - parts weigh next to nothing and the power ability is there as needed + if done right, you can leave most everything on your bike (wheels) stock, may seem silly but first time you try to change a flat on a folder with a hub motor... you get the idea.
As far as folding goes...
All my folders are folded daily and usually many times a day... If I run to the city (Philadelphia), I take my bike to a local train stop... fold it down and get on... at the other end I unfold.. Most of the time if I am going in somewhere for a while like a coffee shop, I will fold it and bring it with me... plus when my children are home, I store the bikes folded (I have too many of them).
The only problem I have with folding is front fender scratching (on bikes I've put a front fender on)...
And your right - it may not be a boardwalk... sounds more like a D7 @ 23 lbs?
-Mike