The Surly Long Haul Trucker is a frame and fork only. About $400 I think. Quite reasonable. But you then have to buy a parts kit, $?, and install everything yourself, or pay a shop to do it. If you've had a 35 year hiatus, you will likely have to buy $100-200 of bike tools to do the installation yourself. Or pay $100+ to a shop to install everything. Or are you planning on putting all of the parts from your flea market Puch on the new frame? Doubt too many would fit. I really doubt YOU will get a Surly rideable for less than $1000. Someone with all the tools and knowledge to swap parts, and a fair number of parts sitting around the house, they could get it rideable for under $1000.
As for using a touring bike for randonneuring, I would not. Believe it or not, you need to carry very little gear with you on a brevet. Even a 1200k brevet. A rack pack on a simple bolt on rack, or even setpost rack, or one of those Jandd big saddle bags, or Carradice bag, or even a medium sized saddle bag, will hold all you need. A Camelbak is very, very useful for drinking and carrying warm clothes. You don't need huge bags or panniers or handlebar bags or all of the other stuff touring bikes can carry. And believe it or not, racing bikes are very stable in the middle of the night after 600 miles of riding. You don't need a very heavy touring bike to not ride off the road as some seem to think. Racing style bikes have triple chainrings and very low gears today.
Of the four road bikes I own, three racing style and one loaded touring, the touring bike would be the absolute last one I would use on a brevet. I would not ride brevets over 200k if I had to use it.