Not going to touch the other items..
I use a 3GS everyday for work. It's a nice phone but dated. With iOS6, it has some nice features, but it still doesn't do everything that a 4S or 5 will do with the exact same OS. I haven't looked up if iOS7 will be available for the 3GS, but I suspect it won't.
It is a small screen, but all iPhones are (the 5 screen is a little bigger), compared to my personal Note 2. I have had zero fatal problems with my phone in the 3.5 years I've had it. It's a computer and like most computers most problems are fixed by a reboot or wiping and starting over. I've only had to wipe it once. I have put a new battery in it, which is cheap from ifixit.com and disassembly isn't horrible, just have to be careful. Most parts for a 3GS can be bought from ifixit, so even if someone claims a button doesn't work of the screen is broke, it can be fixed through ifixit for parts and fixing it yourself.