So hypothetically a phone with WCDMA 850 should be able to get 3G speed with Belus?
Only in areas where Bell/Telus uses 850. In some areas they use 1900, and in some they use both. So you'll get partial coverage, and the only reliable way to know where is to test.
I'm just wondering what the difference is if it specifies HSPA+. Wikipedia kind of confused me because it lists Telus as having an HSPA+ network and nothing about WCDMA. After spending way too long reading about this, my understanding is now that WCDMA is the network and HSPA+ is the technology to make it faster?
Not quite. HSPA+ is a faster version of HSPA, which is a set of protocols (specifically HSDPA and HSUPA). WCDMA is an air interface standard. They refer to different things, but the differences are highly technical and, for your purposes, 100% inconsequential. When it comes to determining frequencies for network compatibility, you should treat HSPA, UMTS, and WCDMA as interchangeable terms.
I don't know how accurate this is but according to Loxcel all the towers in my area for Telus are 850's.
That is based on Industry Canada's Spectrum Direct database, which in my experience is absolutely not reliable. Don't count on that being accurate.
It would appear the Nexus 5 also supports the 2G CDMA that Bell & Telus use.
That version supports the technology, but is missing the 850 frequency required for Bell/Telus, and its CDMA radio can only be activated on the Sprint network in the US. You may not have that variant though; there is both a US variant (with CDMA) and an international variant (without CDMA). I don't know which one Google sells to Canada.