1. Can I use the Uno as an ISP
Yes. At a certain point you will need to disable auto-reset. For the Uno this is accomplished with a capacitor.
or do I need to buy an ISP?
That is also an option but is not required. The Uno will work fine.
2. I've located the hex file for my sketch. Is it sufficient to load the hex file onto the ATTiny using an ISP (Uno or otherwise)
No. The dot-hex file is specific to the ATmega328 family of processors. Your Sketch needs to be rebuilt for the ATtiny2313. The first step is to locate and install a "core". I recommend this one...http://code.google.com/p/arduino-tiny/
or do I need to load the Arduino bootloader as well?
No. If you are using an ISP there is not need for a bootloader.
The ATtiny2313 really does not have enough Flash available to make use of a bootloader.