I'm going to have to correct the last poster in the fact that there are two different ways to go about designing the main body of the full fur suit.
The one-piece body is the more traditional way to go about the full body, and has to be striped out of to use the bathroom.
The split suit is a design where the feet, legs, torso/arms, hands, and head are all separate pieces. This style is just like normal clothing, and can be used as a partial suit or a full suit. Using the bathroom is just like with normal clothing, unless there is a lot of padding in the legs (The digigrade legs an example).
Building your own requires a lot of time and dedication to the task, only paying for the materials, and a lot of trial and error on some of the design aspects (Mostly in making the templates). It's cheaper to make it yourself, and you are responsible for making all the decisions about how to make it, but you end up having to make all your templates from scratch.
Designing a fursuit you can have sex in is possible, but a lot of Fursuiters will dislike you if you talk about that aspect with to much detail.