I'll chime in with a few additional suggestions/points:
- If you buy a commercial theme, and it has features like a "slider" or some other bells & whistles, be aware that many of these contain third party scripts/code that isn't necessarily included in the cost of your theme when those components are updated. For example, a popular slide script is Revolution Slider sold through CodeCanyon. There are a number of themes also for sale on CodeCanyon that contain Revolution Slider. When you buy that theme, you're buying it with the slider as is. If the author of Revolution Slider releases an updated version, you will typically have to buy your own separate license for that to get those updates too.
- Many of the premium themes are shown with a bunch of filler content to make them look good. If you don't have enough content of your own to fill out all these sections, the layout may not look quite as good when you put it up on your own site.
- There are also theme frameworks, such as Genesis, Pagelines and Headway Theme, that will let you create your own custom themes easily. The nice thing about these is you can create as many themes as you want from one purchase.