As long as moisture doesn't have it's way with it (rust on the body, brakes) then an 1800 is probably one of the cheapest vintage cars to drive. They hardly ever break down or wear out, and parts are as reasonably priced as you can expect for a 30+ year old car. Driven non-abusively and given the proper maintenance you shouldn't drain much at all from your wallet.

Like all classic cars, however, saving to much money on the initial car purchase will cost you more in the long run. However, your price range should put you into a pretty spiffy 1800. Not some perfect thiing rolled out of a museum, but certainly above the range of rusty or worn out 1800s.