There are 2 flavors of XC - XC Classic and XC5
If you are just starting with XC I would say start with XC5. It is the new platform and it is here to stay.
On the other hand XC Classic, aka XC4, is way more mature and has way more 3rd party support, modules, functionalities, etc.
So it just depends on your business and business needs. If you are just starting with XC there is no right or wrong direction, it is a matter of what you need and how fast you need it.
Both provide integration with many payment gateways out of the box so there is not much to do but install the stock payment module to integrate with the payment gateway of choice. Keep in mind though some payment gateways will require the use of payment application if CC payments are to be taken on the site (meaning to show CC form on the checkout page).
For sure for PP Pro you will need to use X-Payments in addition to XC.
PP Standard for example you can use directly with XC.
For both you cannot just take any theme design and copy/paste it on site. It will have to be integrated and in most cases you will need a web developer that knows XC to do that not a web designer.
Hope that helps.