Dan:
The 300ml HP772 cartridges can be used. Shop around for the best prices. I've bought them for as low as $105. Lately they have been slightly higher between $115-$130.
Take the starter cart out just before it's dead empty, (I've never used a starter cart before, so the chip might act differently) cut the paper label that keeps the cap (with chip) on the cart and pull the cap off. Do the same with the 772 cart, and transpose caps. Insert cart in printer. (The doors covering the cart will not close because of the longer cart. Should be good to go.
Your cart status dialog will no longer work properly, so you will have to periodically check how much ink remains in cart by pulling the cart out and lightly shaking it, and or weigh it.
The Lt Gray ink is most used whether printing in B&W or in color.
The gloss enhancer is not available in the 300ml HP772 carts.
Enjoy the new printer. I hope to be ordering mine today.