Time for an updated entry, as the cheap flash landscape has changed considerably in the last five years.
As regards the small softbox, see: Are small on-flash softboxes useful, or a gimmick?
I'm also moving the budget to $200, since that's more where the "used OEM vs. 3rd-party TTL flash" tipping point seems to be. At $150, as Rowland mentioned, used OEM models are liable to be the less-powerful smaller models. At $150-$200, you can pick up a mid-range used OEM flash or a 3rd party TTL/HSS capable flashe, as well as rock-solid manual-flash alternatives you can find new from reputable sellers.
The first thing you want to consider is how you plan to use the flash, what features you want, and how much reliability means to you. With a flash, you can have two of any of the following three things: a low price, an extensive feature set, and rock-solid reliability. So, with a lower budget, your choices are an extensive feature set with ok-to-terrible reliability, or a simpler flash with fewer features that you can count on with a client breathing down your neck.
If you're not familiar with flash features, you may want to take a spin around these two Q&As:
What features should one look for when selecting a flash? and What are the most important features to look for in a low budget hotshoe flash and why?
Most of the feature set boils down to whether or not you plan to use the flash on-camera or off-camera. On-camera usage typically entails chasing subjects around at an event. You are moving in and out of dynamic lighting conditions, and you may not have time to fiddle with and adjust your flash output power and your go-to technique for diffusion (unless you don't mind wearing an umbrella). In this case, TTL is probably a must. However, if you plan to use the flash in an off-camera studio-type of setup, then possibly you can give up TTL, HSS, and wireless commanding and go for a manual-only flash.
The lower-cost offerings with TTL/HSS, etc. would include flash units from Yongnuo, Pixel, and any number of no-name Chinese manufacturers who sell on eBay. Higher-cost TTL/HSS offerings would include Phottix, Godox, Metz, Nissin, and Sigma. Higher-cost, but rock-solid Manual only would include Godox and Lumopro. Models are constantly being introduced and retired, so mentioning a specific flash model would cause this post to go out of date as quickly as the 2010 answers here (e.g., a Vivitar 285 is no longer considered a good recommendation, since we know they're made by Sakar (i.e., the Strobist now calls it junk), don't swivel, are missing 1/8 power, and have a proprietary sync connector), and all these newer brands and models have arrived. The Flash Havoc and Lighting Rumors blogs are a good way to see what's out there in new stuff.