the wiring is easy… mechanicals are what would be a bit of a challenge
something like this:
http://cgi.ebay.com/10-Magnetic-BOX-PISTON-PULL-Solenoid-SOLENOIDS_W0QQitemZ230098261894QQihZ013QQcategoryZ55826QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
i remeber a guy on the R1 forums took some bimmer HIDS and made housings for them using some can of somesort. looked amazing when finished. hit me a pm and i can find the link for you if you want
thoughts on this setup?
I skimmed the rest, but they do make high/low single bulb HID setups, they work via a moving reflector to change the beam pattern, pricey for a good one. A cheap kit will end up with broken parts in no time.
theres a thread on homemadeturbo.com where the guy is selling mccullough HID’s for reasonable prices… like 250 for a set of 2… check it out… imo they’d be better than the ebay one 
most motorcycle reflectors are not designed the same way as in a car so those kits probably wont work
If you are going for top dallor, stick with OEM ballast/bulb they are so much better then aftermarket stuff, what kind of bubls is in the 636? 9007?
I suggest finding a cheap headlight for your 636 and retro a pair of bi-xenon projectors, you will love the cut-off and everything. dont buy the lame aftermaket hid kit. they are a waste. oem with color mod owns!
I have had my mccullough set up for a while, no problems, and a few others I know have had them for a couple years now on their cars, no issues, used daily. On a bike… wayy less use.
I think it would be cool to retro a projector in the housing. Keep an eye out for a smashed up car you can snag a headlight/projector and ballast out of 
Jeller
I have HID’s in my R6 and I love them. I got one of those cheap kits of ebay for around $130 shipped. It arrived in like 2 days! I love them. Kinda tricky to find places to hide the ballasts, but other then that, very easy to install. Nick