what costs can be rolled into a mortgage?

Depends on the loan type for specifics, but in general, most loans allow normal closing costs to be rolled in, at least up to some extent. FHA and VA streamline loans without appraisals only allow the VA or HUD fees to be included; conventional loans do allow closing costs to be financed within program limitations. There may be loan to value considerations that cap loan sizes as well (costs are $3000, but financing them would put the loan over 80% loan to value, etc)