If I may ask, how good the Augusta is? Can it carry nukes?
Well it cannot.
Agosta could only carry torpedoes and Exocet anti-ship missile and since this submarine is French, there is no chance of pakistan modifying it in secret to provide it with capability to fire cruise missiles.
+ Pakistan as of now does not have a missile capable of being launched from a submarine.
Even if one day they get that capability, it would be very unwise to put nukes on sub as India has been supplied with 12 P8i Poseidon which is a mortal threat for any submarine force.
Coming to the question at hand.
Since you are not buying from russians
( amur and s-1000 are good sub, though ) and US does not manufacture them,you are left with a choice of only French and German subs.
But even an important question would be; why do you need subs at all?
Job of an SSK is to protect littoral waters and go head to head against surface ship or other subs; it cannot interdict traffic on high seas as it lacks range.(theoretically it could do that but surface vessel for most of the time)
Now
Only countries against whom Saudi Arabia could use these Submarine are Iran, Pakistan and Part of India. Rest of the countries do not have a Navy to speak of.
Out of them Pakistan is pretty much guaranteed to not go on an offensive against you and even if they do, Pakistan Navy is in a pathetic shape. It is a product of step motherly treatment that PN has got at expense of PA and PAF. They do not have even have a Destroyer in their navy.
India has too big a coastline to cover and even at it maximum range, an SSK would not be able to cover more than 20% of Indian coastline and that too in Northern Arabian sea.
+ Indian navy is too big and too strong for Saudi's.
That leaves you with Iran.
The problem with fielding proper subs against Iran is that you will not be able to utilize them properly due to bottom topography of Persian gulf.
so why not buy submarine hunters like Poseidon and Orion and frigates.
The advantage of having surface assets like frigates is that once you are done with Iranian Navy, you could use them to enforce blockades and bombing Iran's coastline which a submarine would be incapable of doing.
Flexibility is the reason that most of Navies prefer surface vessels over submerged one's.
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