For our uses in stunt, especially for control horns, a low temp silver soldering will be more than strong enough if done properly. I use the Stay Bright products available at the hobby shop with great confidence and success. it's low silver content, but pretty high strength and reasonable in cost.
I hate to be a naysayer because there was so much good information in the OP, but Sta-Brite IS NOT sufficient to hold the upright on a plain wire control horn without some additional modifications to reduce the load on the joint. At least not with the uncontrolled conditions that modelers are going to have. It will hold for a while but, like R/C clevises, it's just a matter of time before it lets go. It's the ultimate nightmare to fix, assuming you get out of it intact, basically have to cut the entire airplane to pieces to replace the flap horn.
It you are going to use Sta-Brite, you need to make a different type of connection that reduces the demands on the ultimate strength of the solder. I recommend Keith Trostle's "key" method, where the 1/8 wire is backed up with a 3/32 wire "key". Done correctly you have to drive the "key" into position so that the upright is essentially rigidly held in place even before you solder it. Wrap and solder, and you *might* be OK if you do the soldering EXACTLY right.
And alternative is the Russian (Belko and others) method of bending the upright from the wire itself. Instead of a single piece of wire, make it in two, and bend the upright right into the material. Then wrap and solder the two halves together. You end up with a wire end that fits nicely inside a ball link ball, and it greatly reduces the loads on the upright solder joint.
I use and recommend a very small brazing wire I get at the local hardware store myself. It is more than adequate for a plain flat upright on a wire horn. It can be heated sufficiently with a propane torch if you are willing to wait long enough, but I use MAPP gas which seems a lot more capable.
And I suppose everybody already knows this from Ron Burn and Derek Moran's treatises on the topic, but don't use music wire, use drill rod. It can be heated high enough to melt bronze or silver braze without having to harden and then re-temper the metal or worry about cracking from hard spots later.
Brett