You will not save any money by building up the bike yourself. You don't have the purchasing power of the large bike companies for components to hit their price points. Additionally, you'll either have to buy specialized tools (like a BB too, for instance) or pay to have a shop install. There goes any chance of saving money. A $800 entry level road bike would probably cost you $900 to build with almost no exception. Plus, you won't get the same warranty/service from the shop since you built it yourself. It's just not worth it when you do the math. Trust me on this!
Used bikes in this price range are tough, too. You can snoop around, but generally people want more than what you should pay. An $800 road bike should go for $400-$500 used if it's in near-perfect condition (as there is no warranty for a second owner). But you might find someone who is willing to part with a reasonable bike. Be careful though...the worst thing you can do is buy the wrong size bike. And a bike shop will help you decide what is the best size!
If you buy a decent ($800-$1000) bike, you'll have it for 10+ years if you want. So save up, get the bike you want, and you'll be happier in the long run. And it's cheaper than buying a $300 bike now and upgrading down the road. You are spending $1000, but it's more fun than a gym membership and if you fall in love with it, it will pay you back ten-fold with decreased stress levels, better health, and fun.