Having done a fair amount of steel welding myself; I wouldn't use this stuff.
To my mind, TIG welding is the way to go for most problems with an aluminum hull. Personally, I've never done any of it, nor could I afford a TIG welder at this point.
However, my way of thinking about this goes like this:
There's a right way and a wrong way to do things. The wrong way is usually cheaper and faster, but almost always leads to more fixes later on. Usually when you REALLY don't want to fix them or more usually when you can't. The right way fixes it for good (usually, nothing is perfect).
A lot of it comes down to knowing what you're doing, or finding someone who does (especially with something like TIG welding or this kind of "brazing").
To answer your second question: Around here for a decent TIG welder it would be about $65/hr. Personally I haven't used one, so I dunno if they'll do 30 min or not.
My advice would be to take pictures next to a ruler or tape measure and show them around see what sort of quotes you get. When you find a few people you think seem reasonable, trailer her up and take her to them. See what they say.
It's your boat and engine. Trust your judgement and do what you think is best.
Best of luck!