Militarily, the Saudis are problematical. They took a small role in Desert Storm and the Air Force has seen some action. They have the best equipment money can buy, and a lot of it. But against determined, well-supported, well-led guerillas in hostile terrain .... who knows. With 150,000 men on the ground, they need tactical advisors, command and control coordination, training, logistics .... not much of which is available home-grown.
Most of their ground forces will probably be from other countries, as always.
One thing I found interesting during the Gulf War, was that the Saudi National Guard outperformed expectations somewhat. When operating near their own areas, the bedouin country boys did not break and break and run.
They will probably mobilize some National Guard to reinforce their border areas, while soldiers of other countries engage the enemy, with some Saudi Ministry of Defense troops and officers accompanying - getting photo ops and some on the job training. Maybe they will surprise us, and start toughening up for the new MAD Max Middle East, beyond the ObamaDome.
If the Saudis send their own citizens over the border, I bet that it would be more to test/train a few of them, and for domestic political reasons. Serious military objectives will most likely be covered by mercenaries - oops, I mean the militaries of allied coalition members.
I am guessing that Egypt is getting the main contract to conduct the Yemen Op.
I wonder if Turkey has contracted to conduct a Northern Front against the Shi’ites? Syria? Mosul?