I just ordered the Princes of the Sun expansion, but am wondering what are my best bets for rounding out the Martell options as far as chapter packs go.

Any that stand out for first-time Martell players? While I'd love to just buy them all willy-nilly, budget dictates I'm selective!

What else do you have of the card pool?

I would say pretty much any of the current cycle will do well for Martell. They've been getting plenty of strong cards, a lot of them focused on Icon Manipulation. The Oldtown cycle was also very kind to Martell.

Martell has the most cards on the restricted list, and pretty much all of them are quite strong. Princes of the Sun will give you one, The Viper's Bannerman. You could pick up The Battle of Blackwater Bay for Venomous Blade, and Return of the Others for Burning on the Sands. There's also Fury of the Sun in Ancient Enemies, but it's one of the weaker Furies; if you're building for other houses, the Fury plots are great, but out of Martell it's never going to see much play.

It's not restricted, but it's definitely one of the most significant cards Martell has right now; Ghaston Grey is in Forging the Chain.

If you're interested in running a Sand Snake deck, you can get pretty most of the core cards for that in just three packs. Of Sand and Snakes has two of the unique Sand Snakes, Nymeria and Tyene, and a non-unique Bastard Daughter. Dreadfort Betrayal has the other Brotherhood-cycle unique Sand Snake, Obella, and No Use for Grief. And Tourney for the Hand has another unique, Dorea, and the event A Nest of Vipers, which are really the cards that made Sand Snakes viable.