Ensure the steps are 737 compatable. The doors on the 737 open outwards, so you need a fairly big area for the door to swing out. If not you have to stop the steps a little bit before the aircraft, open the doors, then push the steps on. Not ideal, or all that safe. Also, ones with a swivel plate at the top where the steps meet the aircraft, as the 737 front and back requires the steps to be angled slightly towards the wings. Until you get this angle cracked, a swivel plate saves a lot of repositioning of the steps.
A belt loader may also be required. The 300 is fairly low, but with a large volume of bags, a belt loader may be preferable to passing the bags into the hold manually.
Other than that pretty much everything else has been mentioned.