The other XCOM: Enemy Unknown - The Complete Edition have expired (or £3.74 without DLCs). This is available until end 6th Feb 2017.

"From the makers of Sid Meier’s Civilization comes a game about civilization’s last stand. As the commander of an elite paramilitary organization known as XCOM, you control Earth’s defenses against a terrifying alien invasion.

XCOM: Enemy Unknown – The Complete Edition includes the Game of Year award-winning XCOM: Enemy Unknown, plus the critically-acclaimed expansion, XCOM: Enemy Within, and all of the released add-on content packaged together for an incredible value.

The Complete Edition Content Includes:

XCOM: Enemy Unknown
XCOM: Enemy Within
Elite Soldier Pack add-on content
Slingshot add-on content
Second Wave add-on content"

Steam says: Metacritic: 88%, Overwhelmingly Positive reviews (95% positive)

The extensive mod Long War is available for XCOM: Enemy Unknown: http://www.nexusmods.com/xcom/mods/88/

XCOM 2: Digital Deluxe is £18.23 (£15.74 on its own):
https://www.bundlestars.com/en/game/xcom-2-digital-deluxe
And Long War's sequel, Long War 2, is now available for XCOM 2 (17 Jan 2017):
http://www.hotukdeals.com/freebies/long-war-2-mod-for-xcom-2-pc-steam-a-must-play-reinvention-xcom-2-released-17-jan-2608284