Deus Ex is only three games. The original was very good. 'invisible war' wasn't well received (and i never played so I can't comment). 'Human revolution' is the most recent installment and got mixed reviews (mainly inconsistent difficulty and balancing issues). As for needing the back story, I would say no. Picking up with HR, you will not lose anything for not having played the older games (IMHO).
Not that I am trying to point you away from Deus Ex, but you should really check out some of the more recent Splinter Cell games first. I think you will find it much more 'in the vein' of the MGS games at least as far as the stealth action you were looking for. There is lots of sneaking around, stealth kills and generally lots of sneaky spy action.
One other thing. Both Deus Ex:HR and Vampire the masquerade (and Skyrim and Amalur) are more in the line of RPG type games. They all have stealth behavior and sneaking around, but are more about building a character (who might be a stealth person, but might not) rather than a full stealth engine like Splinter Cell or The Thief series.
Just my opinion.