I think you'll find that most G scale equipment will work fine with 5' diameter curves, in fact most train sets actually come with 4' diameter curves. I wouldn't run a UP big boy on that since it's 5' in length but some 2-4-0 or 2-6-0 steamers would look great.
As far as where to get it, I think you'll find that SPTrains.com has the best prices around, I've also had good success ordering from wholesaletrains.com
Here are G scale Steam Locomotives
Passenger Cars
I'm in the process of getting our G scale train sets up on the site. That might be a good way to start so you get some track, a few cars and then just need to expand. I hope this helps, if you have any questions just let me know and I'll help you as much as possible.
As far as where to get it, I think you'll find that SPTrains.com has the best prices around, I've also had good success ordering from wholesaletrains.com
Here are G scale Steam Locomotives
Passenger Cars
I'm in the process of getting our G scale train sets up on the site. That might be a good way to start so you get some track, a few cars and then just need to expand. I hope this helps, if you have any questions just let me know and I'll help you as much as possible.