Hi just to add to what Jake has said...
The client address fields (Address collector) can be edited by editing the following template in the Backoffice...
BO > Utilities > Templates > Payment > Checkout > checkoutaddress.template
You can make these fields required, there should be code (tags) already there to show you how.
The only thing you can't really edit is the email input.
Most, if not all button and label text can be altered by editing the language files in Dnn ( I would change them only on portal basis)
Dnn > Admin > Languages (choose to edit 'Site" for the language you are using.
Then go to > Local resources > DesktopModules > NB_Store > App_LocalResources then look for the relative resource file where the text you want to change.
To hide the VAT go to your checkout module, settings the go to the nb store tab and look for "Hide vat code field", check this and save.
As for the payment part I would use "Pay by cheque" but call it something else like "Click here to finalize your order".
I hope this helps you further.
Geoff