Agreed that smartphones are a more viable option than tablets. (People love the tablet idea, and it will work, but phones are more practical). The form factor of the phone is great for data collection. Tablets have the one advantage of being easier to type on, so if your survey involves a lot of qualitative data collection, that is an option.
The most important rule is that the phone must be Android. You can get Android phones in many varieties, and all of them will run KoBo Collect. You will want phones that have no contract attached (you don't want to get a phone that requires you to pay T-Mobile $80 a month.) You also want a phone that is Unlocked. Unlocked means that you can use it on any network. Many phones are sold to be used on a particular network (like T-Mobile or Sprint). Any phone that is locked can be unlocked, but this is a pain in the neck. So, buy an Unlocked, No Contract, Android phone.
Inexpensive Android phones are available that have no trouble running KoBo and collecting data. We have tested very reliable $80 like the Huwei Ideos. If you are looking for an inexpensive device to use for data collection, there is no reason not to consider these.
Some notes on buying phones for data collection:
1) Check the phone's Specs. Make sure it is an Android phone and runs some version of Android greater than 1.6.
2) If you are going to be collecting GPS locations in your surveys, be sure that the phone has GPS. Few smartphones are sold without GPS, but you have to check.
3) Check to see if the SDcard is included. Sometimes, to keep the price low, very inexpensive phones are sold without an SDcard. If this is the case, just buy an SDcard along with your phone, they are very inexpensive and any micro SDcard will do. Again, check the phone's specs to see what kind of SDcard it takes. Almost always, this is Micro-SD.
There are a number of sources where you can buy these devices. In fact, you can buy used ones pretty cheap, so I include a couple links here for marketplaces where you can buy Used Androids:
Top Recommendations:
High End Phone
Nexus S
http://www.android.com/devices/detail/nexus-s is a great
phone that exceeds the requirements needed for data collection, while still being a device whose price has come down since it is more than a year old. You can get these from Amazon for about $350. This is one
phone that does not need an SDcard, it has internal memory. It works great with KoBo Collect.
Inexpensive Phone:
Users have reported good results using Huwei Ideos, an inexpensive smartphone. You can get them from amazon, unlocked and no contract for about $130. Cheap, but they work just fine in the field.