You can get a Voigtlander level and a two for one accesory shoe adaptor that lets you add the level and the viewfinder at the same time to the accessory shoe on the top of the camera. But....and this is a big but....using the level will only result in getting perfectly level shots straight on. If you need to tilt the camera up or down to get the view you want, then the level is of no use.
In the case of "restoring" the perpindicularity of the lines, if you scan your negatives, you can use the transform function in Photoshop....you select the image (drawing a selection rectangle around it), then under the file menu, select transform...perspective.
You then point your mouse arrow to one of the upper corners and either push in or pull the image out until the vertical lines are straight. You would then hit return, to make the changes stick, then recrop the image into a rectangle. I do this quite a lot, really myself. Sometimes you have to first deal with the horizontal lines of the image being parallel to the horizon line of the edge of the print. You would first rotate the image to achieve the horizontal levelness, then go through the transform, perspective routine to level out the vertical elements of the image.
Of course this results in a trapezoidal photo, which you then recrop down into a rectangle.
If you scan your image at the highest resolution available to you, which is usually 4000dpi, and at 16 bit, even after all this giggery-pokery, you will still wind up with an image that has enough quality to make big, big print. After all your manipulations, which would include density corrections and possibly dodging and burning, you then change the mode to 8 bit before you save to reduce the size of the final file at no loss of quality.
