I'm not a massively huge big fan of vehicles in FPS games, unless they are done really well and feel relevant. If they are included in S2, I hope they don't become something you feel you have to rely on to get around, ie Far Cry 2. They should be used sparingly and maybe require expensive and hard to find fuel to run. Maybe you should have the ability to push any empty vehicle towards a slope and then hitch a ride as it runs downhill? A major quest could even be created around the idea of fixing an incapacitated vehicle so you can use it to reach previously inaccessible areas?
Female stalkers will naturally add an exciting new dimension to the Stalker universe and yes, of course sex would play some kind of part in it. I mean, a dangerous and bleak landscape, vodka, men and women; perfect recipe for some romantic sexy time. да?