Need major help. First time portrait shoot.

I'm the relative with "the good camera", and have therefore, been asked to take a senior picture of a future HS grad. If taken outside, or with natural light, I'm confident in my skills. And I will be taking those. But shots taken indoors, with flash, not confident at all. In fact I don't even own a flash.

So my first concern is, what flash to get (for T1i). I want to spend the very least possible on this; as I probably won't use it for other than this venture, unless I find it has some future. I have looked at those $50 units. But they seem to only shoot at FULL blast. So I'd like at least some basic control on the unit itself. Or, is it best to simply try and make all flash adjustments via the camera's ISO, aperture, and shutter speeds manually. This scares me a bit. I will not see my results until the shoot is over; with little to NO chance to reshoot if there are problems realized after the fact.

Then too, I happen to own a flash bracket from my 35mm days, but no sync cord, which I'll need. Do flash units ever come with sync cords? It would need to by one that attaches to the camera's hot shoe. I'd VERY much like to solve my equipment issues for $100 to no more than $150. If more $$ is needed, I'll suggest taking 100% of shots outdoors. Can these inexpensive speedlites ever work as remote flashes, being triggered by the camera' built-in flash? Or is this something I shouldn't even consider. I will have some limited time to practice on my wife. (She'll love that, I'm sure.)
Finally, I plan to use my Tamron 17-50 2.8 and/or Canon 50 1.4 lenses for this.
I eagerly await your suggestions.
And, thanks in advance.