As it is now, that core is useless for use as buck converter.
if you want to experiment with it, you might try and unwind it, and roll it back up with a strip of paper in between each layer.
I can't tell how thick that metal is, but you might need to use something as thin as a VHS tape to space the layers out.
also, i don't know how to estimate what the permeability will be if you do that.
as it is now, the permeability of that core is probably more than 5000, and it is a 1 H inductor, but only up to like, maybe 10 milliamps?
50 microjoules of energy storage i'd guess.
try and find the saturating flux density, using an ac signal with a capacitor in series so as to avoid any dc bias.
i suggest you use my choke calculator spreadsheet here
http://johansense.com/bulk/spreadsheets/use the open office version.
calculate the energy needed to be stored in the buck inductor from sheet 4, as well as the peak current.
then use the other tabs to find an appropriate size of inductor.
remember these key principals:
energy stored is proportional to flux density squared
energy wasted in copper losses is proportional to flux density squared.
energy wasted in the core is generally proportional to both frequency and flux density squared.
energy stored in the core is almost always zero, it is stored in the air gap.
copper losses are proprtional to air gap volume squared.
be reasonable with the copper fill factor. 30% is about all you can get for a machine wound toroid. 60% might be reasonable for hand winding.
if weight is real sensitive you might try aluminum wire.