You want a pre fermentation and a post fermentation hydrometer reading. After about a week you will already be better than half way through fermentation, so you won't get much info from this. It would be ok to start the brew without the reading, but you would not know the exact ABV of the beer. A hydrometer reading at the end, a day apart, will tell you fermentation is complete and the beer is ready to bottle.
As for a blow off tube, I took a chunk of tubing that I had for a spare syphon and stuck that through the bung for the airlock. Seemed to work pretty good, let the gas and any foam pass through it into the water bucket. A bit of a challenge to get the tube through the hole, but now I am going to leave it and have a premanent bung and blow off tube.