Someone will need to login to your PayPal account and create the buy now/add to cart buttons for your shopping cart pages.
These are only generated through the PayPal interface once logged in. If you don't feel comfortable giving out that information to your developer, you can create the necessary buttons yourself and send him the resulting HTML code (or hire a developer you can trust
)
With PayPal as the ONLY shopping option, you will find customers that don't mind it at all and are happy that you accept paypal and you will find customers that specifcally won't shop at your site because they don't like PayPal for their own reasons that may have nothing at all to do with your company.
I don't like the functionality of the PayPal cart (how it pops open a separate window when you add a product to the shopping cart), but it wouldn't stop me from buying a t-shirt I like when I see one. That's probably because I'm a t-shirt junkie though, I don't know if all customers will have the same patience.
You can accept PayPal and still use a free shopping cart option like oscommerce, zen-cart, cubecart. That way you'll still have a better shopping cart functionality (no popups) and be able to upgrade to other payment options easily later down the road.
Also, If you use a shopping cart option like the ones I mentioned, you also wouldn't have to give your developer access to your PayPal account. Then all they would need is your paypal email address to plugin to the shopping cart application and the shopping cart would handle the rest.