Changing from a tub/shower to shower only will require changing the drain pipe size from 1-1/2" to 2". Cost depends on what materials you choose to install. Since this is not a job you want to redo every few years, I suggest using quality fixtures. Doing it yourself is a great way to save money, but if you only have one bathroom, it could be a real pain. You need to decide between finances and the amount of time you will be without the tub/shower. Take a serious look at your DIY skills and decide if you can do the job properly the first time since this is not something you want to be fixing continually. I love DIYing and it is great to look at a finished project and take the credit for doing it. Not so nice if you are taking the "blame" for it.
if you have never made a mistake, you probably haven't done much.