Kittay,
I recomend you buy two books by Tom Bailey, follow the link below to his website.
http://motyeliab.com/otm.htmlthe buyers guide will probly answer most of your general questions, and will go over different models and Ducati history in general
it should be quite an interesting read.
the other book is mostly a workshop manual, and it has the widecase supplement and service bullitens, and some other added information all quite usefull.
Ducati used a few different forks over the years, the most common are the 31.5mm forks, that would be what you would have on a narrow case 250 scrambler
and a NC Monza as well. (and a lot of other models)
Ducati used a lot of different headlights, they varied by shape, size, design, mostly changing due to what they needed to contain...ie. switches, gauges, wireing ect.
I wouldnt concern yourself with that too much just yet.
as far as wheels goes, generally scramblers came with a 19" front and an 18" rear. road bikes generally came with 18's front and rear.
most of the hubs Ducati used for singles are basically the same.
If you post some pictures of your project we can identify what you have for you.