I note that you said "spend as little as possible." Good - you might consider "postponing" investment for a while... put 600 or so more miles on the bike thinking about what you'd like to change before dropping any serious bux, because your priorities may change in that time. Maybe you'll start to wish for a different saddle, maybe better shoes or a computer. New wheels & tires will make you somewhat faster, and you'll hear that "wheel upgrade is best bang for buck," But if you're in it for the workout, watts is watts. Fit and comfort may be the difference between 2 hours a week and 6 or more. New wheels will be a significant investment (at least $400, prob more) by the time you're done if you're spending wisely.