Chromed steel Radelli rims, 36 hole, 18" WM0 I think? Maybe WM1? Virtually everyone building specials these days goes with alloy WM2 rims, Excels are popular if you're not picky about a big ugly red logo on the side.
You can use the front and rear hubs from any narrow case 250, or 175 or larger actually. You probably want something later rather than earlier though, something that uses an integral speedo drive.
You might want to pick up a few reference books. Mick Walker's "Ducati Singles Restoration" and Ian Falloon's "Standrd Catalog of Ducati Motorcycles" are two of the typical books, but there are a bunch of them out there. And like I mentioned before, if you're going for accurate (an accurate fake?), be sure you bring your checkbook.