Howdy, and welcome to the forums! I'll try to answer some of your questions as best as I can. I'm assuming you want to replace your head unit with an indash GPS, so that's what I'm gonna write about
First, I suggest you take a look at:
http://www.nicoclub.com/articles.php?id=294580. This is our resident audio genius' (rjdmmfl1) write up on how to install the Pioneer Avic D3 in an 08 Altima Coupe w/ Bose. The only thing that'll really be different in an 05 is how you remove the panels and such, and the wire colours may differ, but installing a new head unit or nav system in any car is basically the same.
1) Any nav system should be compatible with your car, it's just basically what features you want, what you're looking for, how much you want to spend, etc. They make the third party nav systems and headunits pretty universal so you can put them in any car, you'd just need to get the proper dash kits, etc.
2) Pioneer is the most popular brand. Take a look at the Pioneer Avic-D3 or the Avic-Z2 or Avic-Z3. The D3s are super popular and cheaper than the Z2s, but it uses DVDs for navigation. The Z3s are more expensive, but hdd based and more features. Also, I would recommend you look at the Kenwood 5120. and 8120. rjdmmfl1 switched from a Pioneer D3 to a Kenwood 8120. You can find out more about the 8120 at:
http://forums.altimas.org/zerothread/331609 and more about the 5120 at:
http://forums.altimas.org/zerothread?id=326433. They're both flash disk based, so you don't have to worry about DVDs and such for them, and both have a ton of features that you can research.
3) For any indash unit the installation will be pretty similar. However, the Kenwood units are probably easier to bypass (allow you to use all the features while the car is in motion), while the Pioneer units need a bit of extra work to bypass.
4) rjdmmfl1 pretty much goes over everything you'd need to install a unit, but the dash kits and such would differ. And if you don't have a Bose system then you wouldn't need the line-out converter.
5) Anywhere! Amazon and ebay will generally get you the cheapest prices on the products, but look around. There's crutchfield.com but that's more expensive, but has lots of guides, information and reviews on products.
6) You'd have to get a back up camera separately. Take a look at:
http://www.crutchfield.com/App...tp=83, and
http://www.crutchfield.com/App...tp=83 for a whole bunch of options. The Boyo unit in the first link is great I hear, but it's listed on the site for $250. You can find the same thing on ebay for $150 and on amazon for $170.
Hope this answers your questions, and feel free to keep asking