If considering cost-effectiveness, if you don't want to buy the outrageously expensive Brompton, I think you can learn about the Litepro S9. This bike has a high cost-effectiveness, with an aluminum alloy frame that is lightweight and portable. YouTube has their in-depth review, and their official website allows you to purchase it directly. There are also more affordable 20 inch steel frames to choose from. They can also DIY modify it, saving even the cost of modification.