The only difference between a FG randonneuring bike and any other randonneuring bike is rear hub and the chain-set. Budget way to go would be to get an old road bike with horizontal dropouts that's as close as possible to what you are looking for and put an old single-ring crank on there with a fixed rear and front dynamo hub. I doubt it will be easy to source an inexpensive wheelset like that so you may consider getting the parts and building it yourself. I run right around 70" most of the time, but gearing is pretty personal. You'll have to figure out what works best for you.