If you require a shower instead of a tub whether it be health reasons or personal preference any easy retro fit that would cost around $2000 to do is this-
Instead of tearing out the whole shower/tub area you can remove tiles to approximately half way up and the tub. This will be filled with new wunderboard or denshield (do not use greenboard, not resilient enough to water for the shower)
Purchase a decorator trim (listello) and use it to divide the old tiles with the new. Choose a neutral tile the same size as what you have or a square one and do it on the diagonal below the border.
There are lots of retrofit shower bases to go in a tub space with the drain the correct location so it is an easy connection for the new drain. You can even get them with low thresholds for handicapped accessibility.
Now, you can either put a longer shower curtain on or purchase a door (approx $400 -$800) for ready made door.