I've been using a Lifespan 1200 walking treadmill with an appropriate height desk for about 2 years and did a standing desk for about 6 months before that. The walking desk is easier on your feet and burns more calories (directly from walking and indirectly via reduced 'resting state' time). Shoes required are very different - standing requires firm ankle support with a multi-layer sole & insole combination, while walking requires high quality shoes that limit your side-to-side foot roll.
Unfortunately, this program doesn't come cheap. You can spend $1500 for a desk & treadmill combination. (Much less if you want a crappy setup that you won't happy with, or with custom-built setup that you create yourself). Insoles replacements can add up, though shoe costs are not a problem long term if you buy high quality expensive shoes to start.
If you want absolute low cost, skip the desk and focus on frequent natural movements & a simple diet of the essentials. A rigorous exercise program can make you very fit, but won't give the long term cardio benefits of walking. Strength training will increase muscle mass that will increase your caloric load, but it doesn't give any cardio benefits at all. Diet only can eliminate the problems of weight and nutritional imbalances, but doesn't improve your cardio.
Combine a desk program with an intelligent nutritional plan (the correct nutrients at the correct time, with the correct caloric load) and you can lose a lot of weight. (I lost 100 lbs in two years.) After you reach ideal weight, you will likely find yourself consuming as much calories as what made you overweight. (If you do this, make sure you will stick with the the walking desk regimen long term for the health benefits, if you go back to sitting again, you just put yourself on the yo-yo diet program.
In case of atomic attack, all work rules will be temporarily suspended.