ok so bought a bike with full campy veloce 9 spd over the internet an old cannondale to find out the dude who had it before me put a shimano cassette on it. Am i correct to assume that a Sram RD will fix this issue for the cheapest price.

insufficient Information .. not much of a clairvoyant.
good luck.

have a Bike shop to show things to?

If everything else is Campy 9, but the rear wheel takes Shimano cassettes and is currently equipped with one, and you want to use Campy, then I'd advise just getting a Campy-compatible rear wheel and a Campy cassette, then sell off the Shimano-compatible wheel. There does exist a "conversion" cassette that uses Shimano-style splines but Campy 9sp spacing: https://aebike.com/product/wheels-man...w4583-qc30.htm But you back yourself into a corner... one manufacturer, one available ratio, and quite expensive.

First put it together and see what you have. It won't index perfectly, but you might be willing to live with it. If, as is likely you're not, replacing the RD isn't the solution. The answer is to match the cassette spacing, which can be done with any loose sprocket cassette that matches the wheel's splines.

The spacers can then be shimmed or sanded to the correct width to make the spacing match the campy system.

It might be possible to sort it by just clamping the cable differently...