The 1815 is a really classic watch, and Langes are built like tanks, to last. You have nothing to worry about with a Lange of any age. Lange will make it good as new when you send it in. Overhauling an 1815 movement costs about $1K, and add another $600 if you want the case refurbished. Just never let anyone other than Lange service it. Note that very old 1815s might not have AR-coated crystals, so if you have a strong preference for AR-coated crystals (like I do), you may want to get the front and/or back crystals replaced when you get it serviced. As others have mentioned, there have been several sizes and iterations of the 1815 since 1995.
See the "Historic Data" tab and "Annual Prices" tab on this spreadsheet for more info.