I'm thinking the Giant appears to be some kind of TT frame.

Is that a clamp on top of the seat post mast? Perhaps some better photos of that. You may not need the seatpost, but it will make fitting the frame more complicated.

Can you estimate your bike fit before starting? Do you have a road bike that you can measure the bottom bracket to seat rails? It looks like the Giant seat can be lowered about 2", and perhaps raised a hair. A TT bike might raise the seat slightly more than a road bike. But, it is a non-starter if you can't get close to fitting your legs.

I don't think I would pay $120 (even Canadian) for that Kijiji bike, to ride or for parts. For a parts swap,there would be much better bikes to start with.

I'm not saying that the swap is a bad idea, but as others have mentioned, look for a more modern crashed donor bike. Or, sometimes one will find something like a 10 year old bike that is underpriced for what it is (brifters, etc). Maybe low end brifters?

Being a TT bike, how do you want to build it up? Road drop bars? Bullhorns? Modern TT bars? etc? That will dictate a lot for your ultimate build.

Expect to hemorrhage some money on the build. $500 in parts (and skimping at that). Do you have a local bike co-op/recycler that would have a chance of having some nice parts?