To answer your question, yes, it's 'enough' to make your Zencart shop work. as long as your customers have some way to pay you, you're good to go.
Whether it's a good idea or not, I think, depends a lot on what you're selling, and, more importantly, to whom.
If you're selling video games mainly to younger people, for example, you might get away with PayPal only. If you're selling high-dollar stereo equipment, no chance.
I have a PayPal account, but only use it to buy one-off products or rare stuff i can only find on eBay (and then only if that's all the seller accepts). If you want to sell me stuff I can buy in a lot of places, you're going to need a proper credit card merchant account.
Our sites are mainly B2B and we don't even offer PayPal as an option - never had a request for it.