I just did several patch jobs, involving framing, drywall install, tape and mud, and finally a knock down texture. Coming from an area of the country where smooth walls are the norm, texture is the only part of the job that gives me fits. I spent weeks talking to contractors who varied from long backlogs of work with a couple of months before they would even come bid the job, to bids that seemed more appropriate to having the entire house re-drywalled. In the end, it was faster and cheaper to do it myself. The final result isn't perfect, but it didn't take me six months and cost me two months salary to have it done.

Depending on what you need done, you can either go with a compressor and hopper, which will produce a more consistent final product, or if you are only doing smaller areas, you can buy the professional grade texture in a spray can (just skip the home DIY grade, as the nozzle on the cheaper cans will be more trouble than they are worth). Practice on a small sheet of scrap drywall until you get the texture you are looking to match. Sheets of drywall are cheap. Drywall mud is cheap. Until you paint the textured drywall, you can always go back and wet sponge or sand off your mistakes.

Any drywall project is messy. Use lots of drop cloth to cover twice the area you think may be affected.