You can use the online caluculator like what some bank provide. Eg go to OCBC site, choose Tools and Calculator.
To rent out, all property has it’s demand. Usually smaller units is easier to rent out and you are getting less loan. It more flexible, (eg.) Instead of getting a penthouse, you may able to get 2 units of 2 bedroom and can collect 2 rental. If one not able to rent, u still have one to cover your loan. Or you can even sell off one.
Shophouses is different ball game. You may need to work out on 30 year lease than 99 or freehold housing. I can’t comment much because I dont own 1.
You may able to find talk or couses to teach you how to leverage on what you have and own more properties. I am quite against this unless we really know the game and know ourselves well. The risk is too high. My advice, know your risk apetite first before enter to the market.