As far as Maintenance, old folding bikes like this need a thorough safety inspecting. Every moving part inspected, lubricated and adjusted. What I usually find is that the bikes have seen very little use and most of the wear is due to corrosion. A few drops of oil in the SA hub, maybe a new chain, tires, and cables. inspect and lubricate hubs, bottom bracket and headset and hinges. Check to make sure that you understand how the hinges works and the safety latch and they are working properly. no missing cotter pin on stem hinge.

There is sometimes play in the stem truss from a worn plastic insert, this can be corrected with shoe goop and drill. See image borrowed from internet.