If you are on a tight budget I would stay away from trying to create a separate shower. I think your current design is not bad. I like the storage that is integrated with the current layout and like the lighting which could be further enhanced. I would add some better general light in the space at the center of the ceiling-it needs to be a surface mounted fixture of some kind that sends light out in all directions. Also the lighting at the vanity needs a similar general light. Those recessed cans as a single source of light at a vanity do not provide the most effective and attractive light for that purpose. Again, because you said you are on a tight budget, I would leave most of the tile in place. You could change the tile surround at the tub only and pick a color to relate to the remaining tile. Also you could find another paint color for the wall to the left of the vanity to add a different accent color. One last thing is that you could add a glass enclosure around the tub and remove the shower curtain.

Consider glass enclosure for the tub area and refacing the vanity / storage unit.

Changing position for plumbing is what cost the most, knowing that, keep everything in the same place and use glass enclosure for shower and bath if you have the space !