Kritter Keepers are fine as long as you heat them and do frequent 100% water changes. I recommend getting the largest you can manage, but keep in mind that the minimum is 2 gallons because most quality heaters are designed to be used in no less than 2 gallons of water. A 2 gallon container will need a cleaning and a 100% water change every 3-4 days, a 5 gallon container will need a cleaning and a 100% water change once a week.
I recommend an adjustable heater like this one: http://www.fosterandsmithaquatics.co...8&pcatid=11368
mini-heaters, heater pads, and pre-set heaters don't work on a thermostat, all they do is raise the temperature 3-4 degrees based on the ambient temperature of the room. This means they often don't heat the water enough, or overheat the water. Adjustable heaters give the user control and will keep the water stable at whichever temperature that you set it. It's a much better investment.
In tanks without filters, a lot of people find it easier to skip the gravel and either keep a bare-bottomed tank, or if you like the look of something on the bottom, use larger river rocks or glass rocks (you can find both in the pet store or in craft stores) instead since it's easy to just put them in a colander and rinse them off without getting them down the drain.