Check out this website to know exactly the tire/wheel combo that best fits your Y62
https://tiresize.com/chart/
https://tiresize.com/comparison/
Basically you want to stay around 33 inches. If you go higher than that (up to 34.3 inches), your fuel consumption will take a hit, and the tyres will rub. For anything higher than 34.3, you will need to lift
You also need to think about the offset. Y62s come with 35+ offset wheels. You can go down to 12+ with wider tyres so you can avoid rubbing against the control arms
Your aim is to increase the sidewall while keeping the tyre diameter around 33 inches, thats why its hard to do it with 20 inches wheels. If you go with 18 wheels, I recommend 275/70R18 or up to 295/70R18. The latter will rub with the mudguards but you can remove them or cable tie them to the back
18 inches OEM wheels can be found in Sajaa industrial area