Like you, I'm never sure if they will fit okay, if the elastic will be okay and not ride up! (Boy legs NEVER ever again! comfy pftt!) So will only buy a couple and then - cause I live an hour away from nearest Big W or wherever, the next time I go in, they don't seem to have the same style. So the process begins again!!
Occasionally if I do find the good ones again, I go all out and buy 7 or so pairs - expensive little things they are! Then the old 'good' ones get recycled to being night knickers and the old 'old' ones get chucked out!