Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
Why powder coat the rims? if they were new they'd be nice and shiny, you'd really only powder coat poor condition ones. In which case just trawl eBay for 750 rims if you want 2.15 rims or 500/550 for 1.85 rims
Go a little wider. I used to run with 4.10 x 19 front and 4.25/85 x 18 rear and they were perfect. Sizes now are 100/90 x 19 and 110/90 x 18. The more rubber contact area the better so the wider the tyre the more contact there is.