I am flattered by what you have said but surely don't deserve such high praise. I am glad you have seeked advice on forum and taken it rather than making a haphazard decision. This is portrayed by your smart purchase, MashAllah. And once again, very happy to hear that the advice has helped you. Thank you too brother, may Allah wish you with the very best as well, inshAllah.
As for engine oil, 20w50 oil is okay for Khyber. But if the engine allows use the lightest viscosity, it will help improve performance and mileage, the difference won't be much but the car will feel lighter on it's feet. Here you can see the recommended engine oil viscosity and more information regarding your car's engine.
https://www.carrepairdata.com/service/repair/manual/eng/suzuki/swift/1_0/1985-1989/g10/993/37/
Yes I recommended someone to use LiquiMoly MoS2 in their car, because their car's engine was noisy. If yours is too, purchase LiquiMoly Ceratec, it is even better than MoS2 but more expensive. It will create a thin layer of ceramic on your engine's moving parts, reducing friction and making it smoother and reducing noise to an extent. Look it up, it has great reviews, much better than Mos2 but more expensive.
As for AC, everything along with the blower might set you back around 35 thousand give or take. For seatbelts you can go to any scrapyard near your city (sultan ka khoo pindi/shershah khi), you won't find them new but with alteration any seatbelts might be able to fit in your car, it will cost you around 2000.
Sorry for late reply, due to coronavirus we have to shut down our business for the sake of our employees, too much work this week.
Best of luck with your purchase bro.