Using old Adobe Photoshop CS on new MacBook Pro?

CS3 is the oldest version that will run on an Intel based Mac. To run anything older, you would have to run Snow Leopard Server in a VM such as Parallels, and then PS within that.

The trouble is finding an upgrade copy that hasn't been registered, or that the previous owner will correctly include an official transfer form with so that the serial number officially belongs to you.

It's not cheap, though really not much, if any worse than older overpriced copies, but CS6 would be your best bet if you're looking for longevity. You would have to get a full verison copy since you can't upgrade from CS (too old, it won't accept that serial number to upgrade from). However, CS6 is a fully 64 bit app, and that's the important part.

A vendor for the RIP we use told me they were informed by Apple that a future version of the Mac OS will be 64 bit only. So unlike now, 32 bit apps will not run on it. 64 bit apps or nothing. They didn't know how far in the future Apple was referring to, so it could still be a few major releases of the Mac OS away. But then your copy of CS3 won't run and you'll be looking for a newer version. And another "however" for that though is a few versions of the Mac OS later doesn't mean even CS6 will run properly under it.

Agreed I'd also recommend skipping CS3, 4, and 5 and look for CS6 as it was the last version before they transitioned to their creative cloud format.

Try looking for new old stock boxed student and teacher editions on eBay as you're not going to be upgrading it anyways.