Getting it all in a single cheap cell phone, with a single cheap cell phone plan. Possible?

You're stuck with AT&T and TMobile for GSM.

another deal for a T-Mo compatible phone: http://slickdeals.net/forums/showthread.php?t=2414483&p=34613033&highlight=bluetooth#post34613033 (I have an alert on Slickdeals for "bluetooth", I'm looking for a deal on a BT headset, it's picking up all the phones with bluetooth as well.)

I can't say enough good things about t-mobile's customer service. Go to one of their stores. You've been with them long enough that they should be falling over themselves to keep you happy. (Here's a tip for users...call t mobile once in a while...they usually reward your loyalty with something free.) I upgraded to my touch 3G, got my son a new phone and added a data plan. I paid about $100 up front and have a $70 rebate credit coming. The cost of my plan only increased $10 per month, because they fiddled around with my plan, basing it on the number of minutes I really use. I have enough minutes, unlimited data and text on two phones. I have to say, I don't know what I did without this phone. I joke that it's my fiance,but really...I want to marry it. I don't work for tmobile but I swear I could advertise for them! Go into a shop, preferably during the day when there are fewer people in the store. (I went in on a busy Saturday and they spent over an hour with me, though.)

check out this deal: http://slickdeals.net/forums/showthread.php?t=2410103&p=34572649&highlight=bluetooth#post34572649. I'm not familiar with the phone but it seems to meet your requirements.

What sites should I scour for cheap used, unlocked phones? Craigslist. g1s are definitely outdated at this point but I use mine for all of the above uses and it is more than I need. You can unlock it yourself if you buy it locked, if you're comfortable with that. Otherwise, just search "unlocked gsm" or "unlocked t-mobile"(weird search term but it'll work) or similar. If at all possible, get the SIM card first (simple mobile is the best deal for unlimited data I've seen) so you can test it when you buy the phone. EBay offers a lot of generic Chinese phones. Depends on your level of comfort with technology, they're generally a good deal.

If I were to shell out for an unlocked quad band GSM phone, with whatever accoutrements, could I then just switch to the cheapest provider of unlimited chat and data for the GPS, keep the same number, and use it in Europe with another SIM card? Well, you couldn't get your data connection from AT&T or Tmo of the USA while you're in Europe using a European SIM. Your data connection is coming over the same signal as your voice connection. And cellular towers don't have that kind of range. There are guys out there who will unlock your phone for you. And there are unlock codes floating around so you can do it yourself. Just google the carrier, phone model, and the word "unlock." But if you get your phone with a carrier subsidy, you're still on the hook for a 2-year contract. You can also search for "unlocked GSM phones" and find independent vendors selling unlocked, no-contract phones for full price (generally in the $400-600 range). Then you can choose between Tmo and AT&T (again, no Sprint or Verizon for you), and see if you can avoid getting under contract with them. They won't make that easy. You could buy a no-contract smartphone from an independent vendor and just slide your current phone's GSM into it. It would work as a phone, but without data, its utility would be limited. And Tmo probably won't let you add data without getting under contract again (not sure). So this option probably isn't to your advantage. The carrier isn't going to give you a price break on your monthly bill for bringing your own phone to the party.

I was in the exact same boat a few months ago. On T-Mobile, wanted GPS navigation on my phone, didn't want to pay for a data plan. I ended up getting a Nokia 5230, sometimes referred to as the Nuron. It has a GPS receiver and stores all map data on removable microSD cards. Using the included Ovi Maps application, you can view maps and search for businesses and addresses completely offline, just like a Garmin Nuvi. It also does turn-by-turn directions. There are maps for nearly every country, which can be downloaded for free and appear to be updated fairly often. The phone was ~$50 after rebate. I think it was marketed as the entry-level smartphone, so I had to sign up for a data plan for a free trial period to get the rebate. Just canceled the data plan after the free trial ended. The phone also has a decent MP3 player with a standard headphone jack, as well as a usable camera. Takes microSD, which stores the map data, MP3's, videos, etc. Only a few complaints. The screen brightness is low and can make it hard to see in direct sunlight. The speaker volume is also fairly low. This was almost a deal-breaker for me at the start, but either it magically got louder or I just got used to it, and it's no longer a problem now. Finally, the phone uses Symbian OS, which can be a bit non-intuitive and sluggish. Overall, I'm pretty happy with the phone. I think all Nokia phones with GPS now include free maps, so you might want to check other models as well. Another option on T-Mobile could be the Nokia E73. As far as I know, only Nokia phones offer an offline map app. Oh, and if you're planning to use the phone for driving directions, I'd highly recommend a dedicated GPS device. I also own a Nuvi for my car, and it's light-years better than my phone would be. Bigger screen, better and faster interface, louder speakers, easier mounting. The phone works fine while traveling and walking around, but in the car it's much safer to have a purpose-built device (unless there's someone in the passenger seat to help).

One other question: What sites should I scour for cheap used, unlocked phones? Again, thanks everyone.

You should be able to port your number to whatever provider you want, but keep in mind you will have to go with a GSM provider. In the US that means AT&T or T-Mobile (or one of their resellers). I'm not sure about other phones but the latest Nokias provide free nav GPS and you can in fact preload maps onto them so as far as I know, you shouldn't need a data plan. You should also be able to purchase something similar for other phones by buying nav programs from TomTom or Garmin.