Hi Bruce,
Ok, well with what you would like to run(4 axle) you could get it down to a 36" radius, but I wouldn't go below a 42" which will give you a 7ft diameter adding on some to allow for what ever scenery you may want to do. Are you planning on an "around the room" style?
As for track, there is Atlas which has nickel silver code .148 rails, and they offer flex track in 40" sections and a selection of curves in 36", 40.5", 45", 49.5" and 54" right now I think darn near everyone is out of stock...Atlas is saying the first qrtr of 2013 for more stock but I think it will be later than that, they also offer 2 turnout options one is a #7.5 and the other a #5 turnout. Their turnouts however do have overly large frogs and large guard rails, both of which can be corrected if you so choose, and San Juan car co offers a detail kit..
http://www.sanjuancarco.com/product_p/5006.htm
Micro Engineering does offer 36" long sections of Nickle silver track in code 100,125 or 148. They do not offer turnouts in O scale. You can order it weathered or non weathered, the weathered apparently has a chemical etching that makes it darn near impossible to flex as it was explained to me in another thread by AGHR Matt. I myself have not worked with the ME yet, but I have some on the way for an area I just can't comfortably hand lay in.
Here is another option for turnouts.....definetly worth a look.
http://www.oscalemag.com/docs/turnouts.pdf