Currently the two gateways that are supported by default in the PowerStore are PayPal Payments Pro and Authorize.NET.
Which one you choose is completely up to you, the integration and use is very similar for the two. One main difference is that if you choose to use PayPal Payments Pro your customers will have the option of paying by credit card directly on the site or by paying through PayPal. You should do some research on both of these to determine which one is best for you.
PowerStore does not support Payments Standard by default.
There is no limit to the number of products that you can add to PowerStore, but there is no support for digital goods with uploading and downloading.