Hi there.
I think you'll find that the RS250 bodywork won't fit an MC18 without some slight modifications.
I have fitted two different set's of bodywork to my MC18 over the years and it's not that difficult.
I have purchased bodywork from ARD - link supplied by previous writer and also from "Race Products"
http://www.raceproducts.co.uk/
Quality of both is good.
In terms of modifications; basiaclly you will need to fabricate some aluminium brackets to hang the bodywork from, using the original mounting points on the frame as a starting point and then cutting / bending / drilling to fit the new bodywork. It's not difficult.
The seat unit is more tricky - but still simple. I removed the NSR seat and used the sub-frame of the seat as a supporting platform for the new seat unit. I used polystyrene and Duck tape to pack out between the seat sub-frame and new seat unit as it's light and easy to work with. I used original mounting lugs with additional brackets screwed in.
One of the drawbacks of owning the MC18 as opposed to the MC21 is that there are fewer "bolt-on" aftermarket parts available. Check out the TYGA website though, they do a fair bit and are a doddle to deal with.
http://tyga-performance.com/site/index.php?cPath=12_15_28&osCsid=0666dcbeae2e4aa29da0d53a4fe4fc46