Aoa. Answers in order.
1)You can go to Hafiz Jee @ Plaza for rpm meter will cost around 5-6k
2)The best mechanic for this car is the one who you trust but I would recommend Tariq Bhai at landikotal , know him for 10 years and the guy knows his stuff and is honest.
3)Suzuki recommends 20w 50 for this car and about 3,6 liter attached is a screenshot from the car's manual.
4)Don't know about Japanese seats but cultus old shape seats would fit in nicely I guess.
5)Power steering works great in margalla , other PakWheelers can guide you better. but i have seen people getting it installed around in 14-15k but i could be wrong.
6) You can get electric windows in it from Water pump market too , Research a lil and youll get the idea.
7) why getting new panels , get old ones reupholster,
by the way is your the cultus interior one?