If you want to stay legal, then you're not gonna get much help from your motor at 200W--most of the power to get started is gonna be from your pedalling, in lowest gear.
I tow heavy loads (various stuff, including a 100lb St Bernard dog in a trailer, and/or dogfood and other stuff in the side pods and/or in the trailer, sometmes up to a few hundred pounds of stuff, plus the bike itself and me, which each weigh on the order of 170lbs (now the bike is about 15lbs lighter I think but haven't hauled anything with it since the rebuild).
http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=235&p=955968#p955968
For me, I used to use a little geared hubmotor on a more normal bike (DayGlo Avenger) to also pull 100lb dogs in a similar trailer, but that motor was still pulling over 1000W (for a "350w" motor) to get up to speed of around 12-15MPH most of the time. That's only on the flats, which is what most of our roads are here. I overheated that motor a number of times doing that, too, enough to melt solder on the PCB for hte halls inside.
It didn't use that much power once I was going, but to get started it was pretty hard on it. With it set up for only it's original 350W it didnt' help me enough, even back when I could pedal hard to help it.
If you go thru your gears so you can shift, instead of a hubmotor, you might have a better shot at using that 200W ot get started from lowest gear, but you will probvably want to do it stokemonkey style so you still have all of your front chainrings to shift around on too (most chaindrives don't, and only leave you the rear gears, which is not always enough for low power assist like your situation).
So, as far as "specs" go, you really already have the most important one: wattage of your system--it's defined by your laws, so whatever they let you have is what you start with. Is that 200W *at the tire to the ground*? Or is it power at the motor itself? Cuz the former would let you have a fair bit of power, and the latter probably leaves you a whole lot less.
If it just means your motor must be only a 200W-rated (continous) motor, then you could probalby peak it much higher for short infrequent bursts, especially if it's going thru the gears to help it out.
Then you would need enough battery to get you the range you're after, whcih also depends on riding style. If you have ot stop and start a lot, it's like having a bunch of hills instead of riding on the flats, because every time you stop you have to climb the acceleration hill again.
That takes a lot more Wh out of your battery and also adds a lot more heat ot your little motor.
What you might want to do is look at adding the assis to the trailer itself, if that's legal there. it'd also mean you can put all the batteries in it, and use heavy ones cuz the bike won't have to carry them. it also means your bike is a normal bike wihtout the trailer, so you can just unhitch and ride off when you like, if you don't gotta haul a load that day.
I suggest looking at my early pages of CrazyBike2's thread, and at my old http://electricle.blogspot.com, for how I did stuff with SLA, brushed motro controllers off old scooters, and old powerchair geared motors (some of which are rated 200 or 250W, though I used 300W+ ones and overpowered those).
