Ideally, the marquise-cut diamond has a length that is 1.75 to 2.25 times its width. That means somewhere around twice as long, so keep this in mind when browsing stones at the jeweler. What you choose will radically affect the final look of the ring on your finger.
The best clarity grade you can get for the marquise cut is SI2, which is plenty for a good-looking stone. This sufficiently hides most imperfections and is more effective at this than most other shapes. The brilliant-cut obscures most of the inner details unless one takes a long, close look, which is unlikely in most settings. That said, you could try to get a higher clarity grade after making sure it fits in your budget.