I have been looking for this also. Sandbox testing prior to going live is an absolute must. I found this useful link:
The only change I needed to make was to add the sandbox configuration directive to config.php. That single entry was sufficient to make things work. NO IPN needed, just a straightforward return to the site after payment completes. After testing that line can be removed or commented-out.
For my needs, having no specified return address in PayPal is useful because I use the same PayPal account for another website's payment processing. Therefore the return is to the calling website regardless.
The (real) Paypal interface can be modified by adding logos, header, etc to provide reassuring branding that matches the website. Finally don't forget that you will need (for the live site) a fixed IP (£10-£20/yr) and SSL (£10-20/yr for basic domain verification). Without the SSL cert Paypal will report an insecure return back to your site and as a customer that would worry me.
The only ugly thing in the entire process is the dreadful-looking confirmation of payment screen together with a continue button. I'd like to see which file is responsible and provide an override.
Hope all this additional info helps.