In the event of Tony's death, Stane would almost certainly have gained complete control of Stark Industries. Recall, he was Tony's effective mentor. He was Tony's friend. Tony had no remaining relatives, and few or no close friends. It's entirely likely that he would have given his three closest friends (Pepper, Rhodie, and Stane) the majority of his belongings. Most likely, Stane would have gained the majority of Tony's controlling shares. Coupled with Stane's own shares, it would have given him effective ownership of the company, as well as control of the board.
Tony was a brilliant engineer, capable of creating wonderful weapons. His latest, Jericho, would have revolutionized the arms industry and landed Stark Industries a HUGELY lucrative contract. All the other weapon firms would have been left (further) in the dust. With that kind of money, Stane could have hired lots of brilliant engineers to replace Tony. None would have equalled him, not even working together, but they wouldn't have had to - Jericho would have been Tony's swan song, and the boost it gave the company would have solidified it's lead on the arms industry for years, if not decades.
In short, Stane set out to kill Tony not for the Iron Man technology, which didn't yet exist, but for all the wonderful toys that already did. Stane saw that the company was about to reach a new peak, and decided to be on top of the mountain when it happened.