I would like to ask some recommendations on upgrading my rear derailer and shifters on a cannondale touring bike. I have sun tour front and rear derailers and would like to upgrade to a new shimano series while keeping the downtube shifters configuration. My rear derailer is wearing out and would like to upgrade to middle of the road shimano components built for commuting and light touring with decent hills. I have read that mountain bike derailers work great for my type of riding but which downtube shifters would work well with them and would I need to change my cassette as well? Any suggestions would be greatly appreciated.