Hi Paul,
If you have a router, it will save you an awful lot of time and effort when rounding the corners of the body sides.
Personally, I would follow Walter's (multisync) guide-lines, and use the 9.6 mm radius bit, using the guide bearing to follow the edge of the panel. When you have assembled the body, you can then sand the edge a little more (as Brian suggests) to get the final shape, as this is what I did.
When the body is sanded, filled and painted, you will hardly notice the slight indescression in the arc, so I wouldn't worry about it too much.
As for buying extra router bits, B & Q used to do a nice selection (in a plastic hinge top case), under their 'Performance Pro' brand for about ?20, should you feel the need to invest. Alternatively, keep an eye on the 'Cheap Three' (Aldi, Lidl, Netto) as they all do sets of 1/4 shank router bits from time to time, which are more than adequate for the job.
Hope this has helped,
Keep Busy,
Regards,
Mark.