Make sure your tank is clean and if not use a remote tank to try to start it. (I would put new oil in the engine.) If it starts you should immediately be able to hear if it has any weird knocking or other sounds that indicate major $ engine work is necessary before you go any further. If it sounds reasonably ok I would then check valve adjustment and ignition timing and synch and adjust the carbs. So far with any luck you haven't spent much money. If the engine sounds ok and the transmission shifts I would then remove the wheels and make sure brakes and tires are ok. If your luck holds out and you adjust all the cables, etc, you may have a driver for several hundred dollars. If that is the case, then you have the big decision as to how correct and pretty you want to make it and the quality of the workmanship you need to make you satisfied. Here is where the big dollars can come. If you are a perfectionist you could spend another $25k. If you aren't too fussy, maybe $5k. And this assumes you are still doing most of the disassembly and assembly yourself.
If you are serious about getting started with this project you should buy the Barrington Motor Works R50-R69US Restoration Manual. It will be the best money you could spend on this bike.
Good luck and let us know what you do with the bike!