The
Iwasaki DE 150W 6500K in a
mini-pendant would be intense lighting for seagrass in that tank. Alternatives would be the Venture DE 150W 6000K which has a similar spectrum, or the
Iwasaki DE 150W 20000K (AQUA2) which is weaker than the 6500K bulb but has more blue spectra which looks better to most people. You could also use PC or T5 for any seagrass I've grown in a tank that size.
I would add live sand and feed the sandbed for a few weeks (pinches of
Reef Chilli or similar) and let your tank cycle before adding fish. You can use artificial plants as cover for the fish until the sandbed has a chance to stratify and sediments accumulate.
In my tank star grass did better in "new" sand. Shoal grass was slower to establish and like more sedimented sand but dominated the star grass eventually. Manatee grass could throw a lot of shoots in highly sedimented sand. In less you plan to fertilize the tank I would wait untill next spring to get grass, myself.
I enjoyed my seagrass tank for the 6+ years that in ran. There are
numerous coral and other inverts that can do well in a brightly lit, sand-dominated tank.