There is no timetable on how long an engagement should be, if you want to marry her then be engaged for 2 or 5 or 10 years. Just get her a nicer ring later when you can afford it. Now that I am married my wife almost never wears her big stone engagement ring and always the actual wedding ring so you could spend the big bucks on that instead.
Promise rings are essentially engagement rings that both parties would feel less bad about breaking off. I was engaged once before getting married to a different woman, but neither of us dated very much and wanted to see what else was out there so we broke it off. We each married the next person we seriously dated after a couple less serious flings.
TLDR: if you are truly going to get engaged someday then just propose. I was engaged without a ring for awhile when I couldn't afford one.
PS: wish I got some sweet Jordans
