Sage Pay Integration - how difficult is this?

Do you have any guides on integration with Sage Pay?

I notice it as an option in the eCart Checkout Wizard and we have the Vendor and Encryption Password for the VPS Form Settings albeit I believe these are for the test environment which we've been given a link to:
https://test.sagepay.com/mysagepay/login.msp

This is a brand new setup so they are using version 3, although I can't see where to verify this at Sage Pay.

When I go into the Orders tab in the eCart Checkout Wizard I notice a "Default - Sage Pay" Orders table preset which I assume will be created into our MySQL database upon completion of the wizard.

Thing is, I don't have the UPS details from the client yet so am nervous about completing the wizard as, as far as I understand it, there's no way to go back into the wizard to add the UPS option once you've run the wizard, is that correct?

Would appreciate some advice on how easy it is to set up with Sage Pay and if there is anything, in particular, I should look out for as it appears that quite a few people have had some trouble with it when using eCart.

Thank you.
NJ