5W - 40 is not recommended grade, I suggest you to move to 0w20 on your next oil change. Mind that 87k is your odo reading, (combined from battery and engine), the engine would be 60% used in this reading so your engine is relatively new.
I have not changed spark plugs since i bought my car (it was 51 or 52k km when i imported), now it is around 72k i think, If you are not facing any issues your plugs can easily last till 100k km.
About gear oil, as per my research gear oil can easily go till 100k km.
For Cruise Control, i dont recommend "Ustaad Mods" from local markets especially that involves electrical connection. I saw on PW some users installed cruise control on their Aqua, may be you can search for those threads.