While it could last much longer, I'd seriously consider at least a new cartridge and trim to match up with your newly created shower. Plus, consider that what's there may need to be moved to mate up with what may be a different thickness wall, depending on how it is built up and the tile you select. It would also depend on what is there now - some rough-in valves (like the Delta R10000) have been around for a long time, and give you huge flexibility on not only the trim you were to select, but also the function, such as single-handle pressure balance, two handle temp/volume pressure balance, or two handle thermostatically controlled, all depending on the trim and cartridge you select for the rough-in. At 13-years old, it is almost certainly at least a pressure-balanced valve (which is the minimum required for a shower in the USA). Personally, I like a thermostatically controlled valve, especially if you take a long shower (or are the second or third one!) since it will adjust the hot/cold balance to keep the temp up as long as possible as the tank starts to cool off.
Last, drywall (with one exception, and some will debate that) has no place in a shower construction. There are numerous ways to build a good functional shower, and personally, I prefer a surface-applied membrane so that the entire enclosure is waterproof right beneath the tile, but the more traditional versions do work. The advantage of a surface applied membrane such as Kerdi from www.schluter.com is that your shower pan becomes much simpler - only one sloped layer, then the membrane, and tile rather than preslope, membrane (folding corners and dealing with the curb) and then a setting bed before the tile with a conventional shower. Keep in mind that in a correctly built shower, it is the structure, NOT the tile, that is the waterproofing/management layer - tile is the decorative/wear surface. You could technically shower in the thing before the tile was on and it should not leak, but would be a mess - don't do it!
You need to decide your construction method, then carefully follow all of the rules for that type. Mixing and matching is usually not a good idea with various techniques. Good workmanship is critical, but it is not overly hard, just detail oriented. You might want to check out http://www.johnbridge.com/vbulletin/...isplay.php?f=1 for some ideas, too.