You say "they can use GPS navigation offline without a cellular or internet connection." You mean this thing gets communications from satellite without a cell phone plan, without a data plan? Please indulge me, I don't even know what a data plan is. I know it's probably a very simple idea, maybe storage on a server somewhere of data I can access with my mobile device, but I've never had that.
So, I can download maps (for free) to a Nokia 520, and use it in a kind of GPS mode without using cell phone minutes? It triangulates my position from satellite signals, figures out where I am without a plan at all?
Now, I presume that to use it as a phone I need a plan, maybe a prepaid plan. I plan on calling my current prepaid plan people (pagepluscellular) and see if I can use my current plan with a Nokia 520 (their website seems useless in determining this sort of thing). I have over $60 accumulated of prepaid minutes, because there's a minimum use requirement of buying at least $10 worth of minutes (10 cents/minute at that rate) every 120 days. Yeah, I haven't been using my cell phone a lot!
Please demystify smartphone activation. What is "activation?"
BTW, you say you would hate to give up your Garmin 2595 because of the HD traffic, but I saw a review (at Amazon, I think) by a guy who said that feature doesn't work where he drives, and he identified that at my area (SF Bay Area). Can't imagine why it doesn't (or didn't). Seems from reviews I'm reading that the HD traffic feature is very hit or miss. It can work great for some people in some places and circumstances but a lot of people have found the results really bad.
I watched a few videos about the Nokia 520. All people with rather thick accents (Indian, British), a little hard to understand at times. It seems like a really good entry level smart phone at a great price. I'm trying to get an idea of what it will do, what I'd need or don't need.
Edit: Just read some more info on the Nokia 520...
http://reviews.cnet.co.uk/mobile-phones/nokia-lumia-520-review-50010925/
Assuming I might be satisfied with the Nokia 520's GPS navigation in my car, I'm thinking I might want to return my Costco-purchased Garmin 2595, unopened ... right to the B&M Costco I usually go to! I should receive it tomorrow. I'm going to consider not opening the package until I make that decision. I don't drive much these days, the Garmin's HD traffic might not work for me in this area. Why have an extra device?
Modern smartphones have GPS modules which do not require any other connection to know where you are. WiFi and Cellular gives you better accuracy but you don't need them. You'll have to pre-cache the maps though, meaning you need to download the maps while you still have internet connection. Afterwards, you should be able to use GPS navigation without any other connections. Accuracy is not the best but should be ok for driving.