Is any of additional stuff for the new Tomb Raider of any use for singleplayer? Tomb of the Lost Adveturer perhaps? (hm, looking at that one it seems to be not worth of anyone's attention)
I HIGHLY recommend that you get the following, all for single player mode: Agility Skill
(fall further without damage) Animal Instinct
(easier to track animals, which are great for experience) Headshot Reticule
(extra experience for headshots!) Tomb of the Lost Adventurer
(short, but comes early in the game and gives you a nice exp. bonus. It's also my favorite of the tombs!)
and also these, if interested: Pistol Burst Pistol Silencer
(I prefer the trusty bow and arrow, so didn't use these much)
The Pistol Silencer is ok, since you only get it very late game by scourging LOTS of crates. And it's a lot of fun.
The rest of the skills, well, if want to just want to make the game a sight seeing tour. The game is easy enough on Hard mode, already. And you unlock the stuff by actually playing the game. Def. NOT recommended on your first play though.
The Adventurers Tomb is onyl like 30mins of gameplay but gives you a lot of XP. For $.50 it's def. ok.
IF you get something see if you like the SP-skins for flavour, as these cannot be unlocked by playing the game.