I think you might need to refine your definition of "smooth" some: for instance, you want there to be oceans as well, but by definition their bottoms will be at a significantly (hundreds to thousands of metres) lower elevation than the rest of the planet, so I guess you meant the landmasses? Also, would it be ok for you if the large "smooth" landmass would be at a higher elevation as a sort of dry steppe/desert highland? In that kind of setup, there would be large elevation differences involved in the transition between the deep sea to the shore to the highlands.
Further difficulties in having an ocean: if the planet has a moon of significant size, you'd get tides. These would wash up material (sand/soil), and since the land area is very flat, you'd get absolutely MASSIVE tidal flats. This kinds of makes it difficult to build anything lasting, which would make habitation tricky, but maybe not impossible. Maybe everything is built on platforms that float during high tide, and the local vegetation has adapted to repeated submersion. I just noticed you wrote that you were thinking you wouldn't have a moon, which would admittedly resolve the tide issue. I recalled a tidbit that the moon would have protected from asteroids, but based on a quick lookup just now, that seems dubious. Other benefits of having a moon have been suggested, however, including e.g. tidal forces helping to stabilize both oceans and the atmosphere, and the same tidal forces may also play a part in keeping the outer, liquid metal core flowing (due to friction against the tidal forces slowing down its cooling process). Having a moon is probably optional, but I'd say it would probably help in forming and maintaining a habitable planet, especially if the planet is supposed to have been capable of supporting life for a long time.
Another major complication regarding smoothness that I though of is weather, which you expect the world to have. If there's any significant amount of rainfall, either the surface would have to be very porous, with a water table fairly deep, or you'd end up with drainage issues, and any rainwater would quickly gouge channels into the landscape, forming rivers. Rain could also result in vast areas of soggy swampland.
Lastly, there's an issue of how did it end up this flat? You can't have active tectonics, or you'd get subduction, volcanoes, and tectonic plates colliding create mountain ranges as well. So the mantle, at least, would have to be cooled down to be a solid. You'd also need a lot of erosion to get rid of past meteor impacts, any mountains that used to exist, etc. Both of these would require a very old planet.
My suggestions on how to make this somewhat plausible:
- Make it Earth-sized, for similar gravity. A high enough gravity is also required to retain an atmosphere; on the Moon or on Mars, gases slowly escape to space
- A rocky, earth-like planet, but with a solid mantle that's been that way for eons, so there are no remaining signs of tectonic activity
- However, to protect from solar winds and cosmic radiation, it would be preferable to have a magnetic field, i.e. the planet needs to have a liquid metal core
- Save yourself the trouble of trying to work out tides etc. by making the entire world a flat desert. Think of the flat areas of Mars, or Tatooine. The flattest areas on earth are salt flats (not counting atolls, since they rise quite a bit from the seabed).
- Maybe there are a few small hilly areas remaining, the last remnants of the largest mountain ranges of bygone eras. Or a couple of small but deep basins with shallow seas in them, if you really still want to have some oceans.
I think that's about it. I can't think of anything else required for a breathable atmosphere, even if it would probably be very thin and dry in this kind of environment. Protection from cosmic radiation is covered. You could still have Tatooine-style moisture collectors (or check out actual-modern day technology, e.g. http://themindunleashed.com/2016/10/this-wind-powered-water-condenser-can-pull-11-gallons-of-clean-water-out-of-air-each-day-for-drinking.html), and climate-controlled (cooled, heated, moister air, etc.) greenhouses for farming, even if you have to import the technology, fertilizer and such. Long-term this is cheaper and more comfortable than just importing all your actual food. Energy could be supplied by wind and solar power, both plentiful (more on that below), plus fission, fusion, or whatever future-tech you may want to use. The local grass could possibly be used as a raw material for fibers (including textiles, cardboard, chipboard...), bioplastics, maybe biofuel if that's needed, etc.
So, moving onto the area I have perhaps the most credentials for: weather (I've studied some Earth sciences in general, but meteorology most of all). Without a lot of moisture to handle heat transport from the equator to the poles, you'd get quite large differences in temperature depending on what latitude you're on. This would also create quite fast winds, at least sporadically if not constantly. On Venus, this has developed to an extreme situation where it has been suggested that Venus used to rotate in the same direction as the rest of the planets, but the runaway greenhouse effect of its dense atmosphere has created such strong winds in the direction opposite to that "usual" direction of rotation that over billions of years, the wind blowing in an opposite direction to the planet's rotation slowed, then stopped, and ultimately reversed it's direction of rotation. Take this with a grain of salt though, tidal locking effects from the sun probably played a part, if indeed it did rotate in the "common" direction originally. Plus for our "desert planet" thought experiment, we have a thin atmosphere, and it would also be further away from the sun, more similar to Mars or a slightly more distant and thus cooler Earth in that sense.
So the winds would be powerful. Have a look at the storm in the movie The Martian for inspiration. Solar power would also be relatively plentiful and above all reliable, if the atmosphere is fairly dry and thus there are little or no clouds (maybe some thin cirrus or such would form rarely). Other weather effects could include sparse fogs, dust/sandstorms, and Mars at least regularly sees snowfalls composed of dry ice (frozen carbon dioxide) near its poles, because it's so cold there. It sublimates directly from its gaseous state to a solid one, falls to the ground, and when the seasons turn/temperature rises, sublimates directly back to a gas again. Thus, if you have more of a "cold Tatooine" than Mars, water-based snow might be plausible in the colder regions near the poles. I mentioned earlier that outright rain would be problematic, but maybe some very light and sparse drizzle might happen on occasion, with the droplets just heavy enough they don't stay suspended in the air as a mist/fog. If this happens, it would happen during a fog, condensing out of the fog itself, not rain clouds.
I hope this was helpful, and provided some food for thought at least.