plastic bottle score

I was at my QFC store yesterday and they have one of those "fill it yourself" bottled water setups. On a whim I started flipping the bottles over and looking at the bottom. About half of them were 7's, but the rest were 1's and 2's! So for about 10 bucks I can get a nice ballpark equivalent to a better bottle and 5 gallons of filtered, UV sanitized brewing water thrown in (ultimate source is our local tap water). They even sell extra caps (screw on) for 20 cents, so I can get one to drill out for a blow off tube and drill the other one out to fit an airlock. Easy swap after the ferment settles down. The 1's and 2's are also nearly opaque. Translucent enough to see the fill level, but still blocking out most of the light.

Any holes in my plan?

You will probably need a blow off tube instead of an air lock since they are 5 gallons.

or only brew 4 gallons

I was thinking about starting out with smaller batches anyway. I rarely drink more than a beer after dinner, so big batches would take too long for me to go through. Half the fun of brewing is variety, isn't it?

When I first started I thought 5 gallons was a big batch now I know its not. When you start sharing with friends and roommates it goes quick.