Paypal payments pro versus simple paypal buttons

I have been using simple paypal buttons on my site. I am approved to use websites payments pro with monthly billing but it is not clear to we what the advantages are. Does the monthly billing work out cheaper. I was also told that I need a formal shopping cart to use payments pro-is that correct?

You should really be asking PayPal those questions, since they're related to the services that PayPal provides.
Website Payments Pro lets you collect credit card payments directly on your site, without customers having to be sent to PayPal if they wish to pay by credit card. It also lets you collect credit card payments over the phone using their Virtual Terminal. You need to talk to PayPal about all the details of the services they offer, as well as the rates and cost effectiveness.

If your site doesn't already have a shopping-cart "store" on it that integrates with Website Payments Pro, then yes you'll need one if you wish to use Payments Pro on your website.

Zencart will give you the eCommerce platfom you require for PayPal WPP... but your existing site will become "redundant". You will have to re-build the site (not that difficult) entirely within the zencart framework, and load all your data (products, info pages, etc) into the database.