You dont NEED either the 35 watt or the 50 watt lights ... its purely a choice as to how much power you want the lights to have. Standard HIDs are 35 watt. The 50 watt HIDs are more powerful (and brighter) than standard HIDs by about 50%.
Its possible that as the 50 watt HIDs draw a similar power to the 55 watt factory halogens they are less likely to cause canbus problems. The factory that supplies my 50 watt ballasts says "Our company is the first one that have solved the CANBUS problem (BMW, MERCEDES, and so on)"
Yes the ballasts can fit under the front beak. I have 2 there but I reckon you could easily build a bracket on which you could mount 4 of them there. For photos, see here
http://www.ukgser.com/forums/showpost.php?p=833876&postcount=28
To give you a rough idea of output:
A 55w halogen H7 bulb pumps out 1400 lumens.
An overdrive 55w H7 bulb (50+, 55=110 etc) pumps out about 1650 lumens
a 35 w HID puts out about 3200 lumens
a 50 w HID puts out about 4600 lumens.
A standard BMW, Audi, Porsche or Merc driving around with 2 HIDs and 2 halogen foglights on (pretty standard setup) is getting 9200 lumens (3200 x 2 + 1400 x 2) on to the road on dipped beam. By comparison a standard 1200 GS puts a mere 1400 lumens onto the road ... no wonder the headlight appears crap in comparison to modern cars.
Modern cars have multiple HIDs and almost 10,000 lumens coming out the front of the vehicle on dipped beam, so its up to you what you go for, but unless you go for considerably more than 10,000 lumens, or have more than 4 sources of light on the front of your bike on dipped beam, then you are not going to appear exceptionally bright.
Note also that your eyes see brightness as relative. If you are at the llights, next to a car that is putting out 9200 lumens onto the road, when he drives off, you will appear to be in relative darkness even with a single 35 watt 3200 lumen HID .... Your eyes will have adjusted to the light levels provided by the 9200 lumen vehicle next to you.
Hope that helps