I replaced the traditional harddrive with Samsung's 850 EVO (like some members mentioned here), because it had lots of good reviews on Amazon. I ended up buying one from eBay, and installed it with the help of a YouTube video. I couldn't be happier. Average boot time is 17 seconds. Photoshop...
Why not clean install? You can transfer your files later on using an ethernet cable for faster transfer. Get a new copy and avoid the copy of unnecessary files.