Check the clean and dirty water tank capacity. If it is too small, you'll be stopping to empty and refill all the time.(mine is about 4l each which is sufficient, wouldnt want less)
Does it have moving brushes on the bottom? If not, then the ability to get in and dislodge the dirt is minimal, and you wont get as good a clean.
If it says water only, is it using steam? If it is literally water only, and no steam, i would be surprised if got out much dirt and grease. Even with Steam only, there are stans that you will struggle with.
What size is the bit of the machine which cleans the floor? If it is an upright/push along, then the bigger the base, the less work it will be for you as the more areas it does at once.
If it is a pull along with a pole and attachement on the end (Like a henry hoover style), i wold warn you that the end is sooo small it takes forever to do a whole room, and you will find it hard work! I got blisters!