I don't know what model Schwinn that is, but in 1974 a brand new Schwinn Super Sport was $137. It seems these "vintage" bikes (as if they somehow get better with age!) cost more than they did when they were new.

If you want a geared bike, don't get a fixie with the intention of converting it. That'll end up costing you a lot more than just getting a geared bike.

You can ride a road bike on short, dry grass, but if the grass is too prickly or if there is anything on the ground with thorns, you may get a flat. Forget about riding on grass with road tires if the ground is wet.

How far is your commute? Do you plan to do it every day year round, or just when the weather is nice? There are lots of nice commuting bikes which aren't too expensive. You might want to start with the Commuter Bicycle Pics thread on this forum to get a better idea of what people ride and what you'd like to ride.