What are some good brands & can a late 2009 27 inch iMac do a usb 3.0 without opening it?

I don't believe iMacs got USB 3 until 2012 models.

Your best bet on your model is Firewire.
You'll find people to swear by specific brands and also say some brands are garbage.

I have a several WD Green Series that have been flawless in external enclosures I have used them in. (Firewire, eSATA, and USB)

Any Mac can use a USB 3.0 device so long as the USB connector is of the "A" type and not the "B" or "Micro" type. It will just operate at the speed of the machine's native USB bus.

I've had a WD Green HD running damn near 24/7 in my Mac Pro for almost three years now with no signs of dying at all, and several Green and Red drives in my external enclosures which do run 24/7 since those enclosures are attached to eSATA ports on my grandparents' DVRs.

I would avoid the Seagate drives unless you have an actively cooled enclosure - the Barracudas especially run hot.