Measure rise & fall time of MOSFET switching - cheaply
Given the paucity of your equipment you may be better off living mostly in simulation world (eg. LTspice, with the most accurate models you can find) and attempting to verify operation by indirect measurements when you actually build something. 

Since switching power supplies (for static loads anyway) involve periodic signals you could consider building a sampler (here is a link to an article), an ancient technique that works well for such signals, however personally I'd be trolling for an older working analog 200MHz oscilloscope.