I guess I could answer some of the subjective sides of the question as someone who has worked on Symbian OS for quite a few years ;) However I won't, I'll try to stick to what I see as some facts and useful information.
The phone in question runs the Nokia Series 60 UI (these days called s60). Symbian phones have traditionally been split into the OS bit (Symbian OS v9.1, 9.2 etc) and the phone vendor buys or develops their own UI. A particular version of s60 will run on a specific version of Symbian OS.
So a good place to start for development on your phone is Forum Nokia:
http://www.forum.nokia.com/main/resources/technologies/symbian/documentation/getting_started.html
This will cover the UI bits and pieces. I should add that UI vendors do perform a lot of OS customisation themselves (with varying degrees of self-control/quality) so differences in API's especially involving hardware can become esoteric.
Symbian itself does provide a decent level of quality of documentation, not up there with Microsoft but better than a lot of open source offerring. A good reference is the Symbian Developer Network:
http://developer.symbian.org
Regarding tools. The standard development environment these days is Nokia's Carbide platform which is based on Eclipse, I think it is fair to say that if you can put up with Eclipse you'll be fine, if you can't... well... command line time for you.
Carbide has on-device debugging and includes the compilers for PC and ARM (GCC-E) targets.
Most app development is done using the Symbian Emulator which will be part of the SDK for your phone from Nokia.
When you are ready to put the app on the phone, you need to look at Symbian Signed:
http://www.newlc.com/new-symbian-signed-processes-are-now-available
A lot of people have had reasonable complaints that it can be a bit confusing, but as a non-commercial developer I believe "Open Signed" is supposed to be the easiest.