Before you think of spending money to buy new gym equipment it is strongly advised that you look at your second hand options available in your area. Gyms go out of business, a man who trained for 10 years is selling his gym, there are countless good deals to be found if you take the time to look.
Dumb-bells or kettle-bells would be your best option, which if you look around on Craig's List one can pick up a pair of 8kg kettle-bells for only $15. A simple pair of kettle-bells will give you hundreds of options to train your whole body, body-part by body-part.
If you are on a limited budget, then save your money by spending it on heavier kettle-bells and rather find a second-hand plyometric box (plyo-box) or you could build one. They are 18" or 14" high with a wide enough surface for both feet to stand or jump.
This box increases your cardio capacity if you work it correctly, it blasts your legs and gets you to your maximum cardio out-put (VO2 Max) very quickly. .
The next investment, also second hand, should be on weights which should never cost you more than $300 for as much as 800 pounds including a barbell, you just need to look around.
If you have money to spare buying a power rack will cost about $500 brand new, so finding a cheap power rack just requires time and patience. Unlike your training which requires total commitment to train outside your comfort zone at least three times a week.
If you invest in a barbell and weights then you need a good bench which can easily be second hand, as long as its sturdy. Obviously it depends where you are but one could purchase a second hand gym for $300 that will get you results year in and year out, if you stick to a sensible routine.
Training at home alone is tough and relies on a strength of character and commitment that is rare. The most successful home trainers that continue to show results are those that keep a diary, they know exactly how strong they are because they train at least three times a week.