I realize we just announced Jagged Alliance: Crossfire earlier today, but if you'd rather not wait for some more turn-based RPG/strategy goodness, you can immediately dive into S2: Silent Storm Gold Edition on GOG.com for $9.99. The description doesn't specifically mention that the Sentinels expansion pack is included, but the "two complete games" line suggests that it is:
The Silent Storm series brings you two complete games that both provide with you a wealth of turn-based combat against the backdrop of World War II and beyond. It’s the year 1943 and you must lead a group of operatives as they assault against the enemy, wreak havoc on their operations, and find out what new power threatens both sides. After the dust settles from the war’s end, you must find and destroy the remnants of the organization known as “Thor’s Hammer.”
Silent Storm Gold features a fully destructible 3D world with ragdoll physics and non-linear gameplay. Play the game your way: engage in stealth and quietly take out your enemies or take the more direct route with a rocket launcher. Choose from over 40 operatives in each game, for a total of 80 unique personalities and skills, and arm them with over 75 different authentic and painstakingly-researched weapons.