Joe...I've witnessed 9 launches from the Cape....3 of them from the press tent...the closest point you can get.
If you can get the 300mm, that will be a godsend. Since it's a daytime launch, and since everything happens so fast, I would use it handheld because you'll be able to use a high shutter speed. If you can get a lens with image stabilization or VR, that would be good. A tripod is going to slow you down, and you'll be doing well to come a way with 5 good shots.
If your 300mm source falls through, you can rent 400 and 500 mm lenses from the internet (eg.
BorrowLenses.com - Camera Rental and Canon/Nikon Lens Rental) for a reasonable price for 3 days to more than a week. This would be money well spent...if you really want to treasure the shots.
Again, you've got a very short window to get the shots, so take several practice shots in advance to make sure you've got a good line of sight and your settings are correct. Good luck.
Please post the results when you get back.
Jim