Sounds like bogus news.
First of all, the DF-31s that were sold by China were already obsolete at the time of sales, they were neutered by the Chinese and don't have any nuclear (or any other) warheads. By now, they are definitely out-of-service.
Second, even if they had been operable it would be pointless to point them to Tehran or Tel Aviv. The ballistic missile inventory of Saudi Arabia is tiny and any fire would be met with devastating response from either of the countries.
Iran is a massive country with a geographically distributed population, industry and agricultural centras whereas Saudi Arabia is actually a tiny country by land use statistics and can be "taken out" with relatively few surgical strikes against its main economical and industrial centers. As for Israel it has nuclear weapons and much more advanced technological prowess and capablities.
Third, the Saudis lack any kind of technology for domestic construction of ballistic missiles of this kind. Once the missiles are fired, they would be out of ammo. The MTCR (which the West, Russia and China all have signed or abide by) prevents sales of long-range or heavy warheads so Saudi Arabia would have no luck getting anything from there.
The only possibility for this missile base - if it is one - is if it would be equipped with Pakistani missiles. That is not impossible but unlikely due to American influence over both countries.