Hamoud Alasalam o Alakum,
Welcome to Karachi, I will find it surprising if as a Saudi, you do not know basic Urdu since more than a million Pakistani expats live and work in KSA.
It all depends which area of Karachi and what segment of society you are planning to visit in Karachi.
All the large hotels have English speaking staff and most business is conducted in English.
Arabic is taught at most Madrasas and a lot of people like myself who have been educated in the west some how fell in love with Arabic and speak intermediate to advanced level Arabic. Please remember the Arabic we learn in UK and USA is Fusha Quranic or Classical Arabic فصحي , this is slightly different dialect than Khaliji or Saudi Arabic but you can easily understand it.
There are also a couple of Arabic speaking clubs where members meet every Sunday and exchange their knowledge and experience.
Please let me know when you are planning to visit Karachi and I will arrange .an Arabic speaking volunteer ( male of course) for a couple of hours a day. 