I picked up a set of Shimano ST5700's on eBay for $110. The RD is just the function of the Shifters it will move the distance the Shifter tells it. Same with the FD. However the rear cog set should match your Shifters. For instance my 5700's are 10spd. I'd get a 10 speed cog. You could go with a 9spd but it's not quite as fluid.
If I'm not mistaken, the R300 was the Cannondale 3.0 frame. That frame had spacing on the rear axle for 7-speed (126mm), not 8-speed and above (130 mm). You can put an 8-speed or larger cog wheel on it, but you have to stretch it to make it fit. This works better with steel bikes than it does with aluminum. I did it for a few years on a R400 and eventually, the frame broke at the drop outs. I was so glad to get rid of that bike -- it rode like being on a rail. You can probably still find 7-speed stuff if you want to put new components on it. Consult Agent 319 for ebay tips. If it were me, I'd toss it and buy a new bike.
JC, the bike I'm riding is a 1993 2.8 R700 and it has a 130mm dropout spacing. I have a 9spd cassette installed and it fit wonderfully. I doubt a 1999 Cannondale would have a 126mm dropout but check your serial # to this website and it will tell you what size frame and what your dropouts are.
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