It's not a common move, but far from unheard of. Typically what I see, here in the mid-Atlantic region, are ranch style homes that are converted to Cape Cods. The process typically involves a framing contractor, a large crew, a large tarp, and a 4-5 day window of theoretically decent weather.
Now for the potential issues. Oddly enough, trusses are not the best option here. The reasons are many, but I'll cover the big ones. First, typically there is a major attempt made to save the ceiling structure and finishes, of the rooms below. This is accomplished by prepping the house to accept new floor joists and rafters. First attic insulation, HVAC, electrical wiring, plumbing, etc.. is removed. Next the roof shingles, roof deck (plywood, or board sheathing) and rafters are removed. The first step in reconstruction is to install new floor joists. In your case these would probably be 2x10s. They are cut to length, then air nailed to the sides of the existing ceiling joists. New plywood decking is installed over the floor joists. Now you have a third floor, and did little to no damage the the existing second floor ceilings. Next, rafters are cut and installed. At that point the whole job for the framing and roofing crew is no different that a new home. You will have a roughly 12' X 24' interior room with 5' knee walls on the long walls.
As you can see, trusses would be more applicable to a new construction situation. By stick framing you are able to adjust for all kinds of obstacles that arise in these kinds of situations. Trusses are computer designed, and built on huge jigs, so that they are all exactly alike. Thus, an engineered product built to high standards. A hundred year old house can be quite the opposite. One potential disaster that comes to mind would be a high center bearing wall. Unless you have a real unicorn, a hundred year old, stick framed house typically has interior bearing walls and short ceiling and floor joists spanning from the outside walls to the interior bearing wall, which is typically located dead center of the structure, and running parallel with the front and back walls of the structure. If that wall ended up at a slightly higher elevation from any point on the front or back wall of the home, you would have a pretty big issue for a truss install. Trusses are typically designed for two bearing points and a free span between them. So it is designed to bear on the front and back walls and now it's "rocking" on the center wall, and you have a big engineering problem, since you bearing is in the middle and not where it should be. If this were to happen on stick framed renovation, it means there is a nearly undetectable lump in the middle of the new floor, that doesn't impact a thing. The same is true for a lot of conditions you might find in a century old home. For example the roof is now off, and your carpenters start "pulling tapes" to see how square and straight the existing structure is. Turns out it's 2" out of square, and the front wall is 1-1/2" shorter than the back. No big issue with stick framing, but a huge headache with the factory made trusses piled up in the yard. A good framing carpenter, doing a stick frame roof in this situation, is a lot like a skilled custom dress maker. He can tuck, twist and massage the new roof to make it fit. There are a few other reasons that I avoid trusses, in general, but I doubt that anybody doing the job for you would gravitate toward them, so I'll save all that.
Now, it on to killing an old wive's tale. Sorry, but there is zero reason to believe that ANY hundred year old building is.
A. Extremely well built, since "that's how they built them, back in the day".
B. A tired old POS that will fall over in the next wind.
The truth almost always lies somewhere in the middle ground. Anything more than 50 years old, or so, in the states, is a total wild card. It could be ridiculously overbuild, and capable of lasting for a few more centuries, or slap together by a bunch of farmers, on nights and weekends, with the dual goal of, "getting it done, it's getting cold soon", and "stop thinking 'bout all that fancy store bought material, there are plenty of good nails in that milk can, out in the shop. Use what you can, and spend some time straighten the bent ones, fer Christ sake stop whining, you think we got city money, Elwood?" Seriously, before codes, and building standards there was some really crazy shit out there. Chances are, 99% of 100 year old structures that are straight and sound can handle a fairly light attic reframing, but it's best to assume nothing.
Finally, don't forget, you need a staircase to the new room. It can be tough to find the space for one on the second floor of a small home, without losing some serious real estate in a bedroom, or at least creating some "interesting" traffic flow. As for the concept overall, I'm not a big fan, and would only do it, if you have no room to do a typical addition. The $150-250 number is bat shit crazy. In a rationally priced market, there is no reason that $60-80K shouldn't give you a simple two story addition.