Yes, the "checkout workflow" option is in the checkout. You can remove the text "payment_" from it in order to remove the payment view from the checkout.
You can add custom fields via the menu Display->Custom fields. There, click on new and select the table "Order" in the "Table" option.
If you want to have both possibilities, you shouldn't remove the payment view but instead, configure both paypal and the "collect on delivery" payment method. You will have to change the name of that last plugin to "Purchase order" and its description. Finally, you will have to go in the configuration under the tab languages to change the text strings related to the collect on delivery plugin in order to change them accordingly.