"Start" sounds best to me, Paolo. "Take up" sounds possible but a little odd. I wouldn't use "start up" with "job". I sometimes hear people use the phrasal "start up" when a simple "start" would work. When they use it this way, they are usually talking about some kind of machine: I need to start up the lawnmower and mow the yard. = I need to start the lawnmower and mow the yard.
Out of all your sentences I think the first is most natural. It's simple and direct. Adding "up" to "start" is incorrect. "take up" and "job" sounds a bit weird to me, perhaps to someone else it is okay.