Welcome kponds. Randonneurs are a varied lot, but generally do not fit the typical "roadie" stereotype. We are a friendly bunch who usually enjoy the company of other riders. You'll find the typical randonneur will go out of his/her way to encourage and help you succeed.
The bike you pictured looks like a fine steed for Randonneuring. So long as it's comfortable and reliable, it will do fine. BTW, most of us don't have $5k bikes! Keep in mind that the minimum average speed for Randonneuring is less than 10 mph (including stopped time), so you don't need a fancy bike.
As far as doing a 100K populaire goes, if you can currently do 50 miles on a fixie, then you can certainly do 100K.
Good luck to you.