If you are likely to be doing a lot of maintenance on your car, a floor jack is a good investment.
How to Choose
Hydraulic floor jacks are defined by the amount they are rated to lift. If you have a 2 ton car, for example, you should buy a 2 ton floor jack. The jack is never going to lift the whole car by itself, but this gives you a safety margin.
Since the jack is what gets the car into the air and you will depend on it for safety, you should choose a floor jack made by a reputable manufacturer. As a general rule of thumb, steer clear of cheap imported jacks.
As important as the floor jack is the stands you must buy with it. A jack stand is what keeps your car in the air once the jack has lifted it. The actual jack—no matter what quality—should never be the sole support of a car you are working under.
Floor jacks are remarkably easy to maintain. To prevent rust, you should add grease to any moving parts or joints and wipe it down with an oily rag before you store it away in a dry place. The only problem that might arise would be a leak around the seal when the jack is under a load. Repairing a leaky seal may be a job you can do yourself, but is probably better done by a professional.