Sorry the only bike with fixed/free/3-speed options costs about double your budget. If you really want to coast, this one won't out of the box. You'll need to buy a freewheel separately. Figure about $25 for that.

http://www.bikesdirect.com/products/...kilott_s3x.htm

Damn, looks like they only have 50cm in stock. I'm not going to guess what size you need, but 50cm is probably too small.

OK I am going to guess what size you need. 55-56 cm.