Hiya Stephen,
Corel draw will do pretty much any work required for decals. The main problems however are getting the 'colours' right. You will have to do all the markings in the 'correct colours' and as such, even if you use hexadecimal codes that are avaialble on the net for common military marking colours, there is no simple way to do that - a lot of trial and error will be needed as no two systems are the same. Monitor colours vary, printing inks vary in density and colour. Once you have a match to what you need, set a 'colour profile' for the particular sheet. I have never tried to do decals with anything other than an ALPS printer as they are the only ones that can print whites (obviously a massive advantage on smaller markings) and they can also print 'solid' colours without the 'dots' associated with other printers. But for inkjets and lasers there are ways around this. Printing on white decal paper being one, or printing on clear paper and using a white background on the model being another. However, there will always be the 'dots' even on the finest printers availble.
I know some chaps here have had a bash, and will be able to help more with the inkjet/laser route than I can, but anything i can do for you i'll give it my best shot.
Rob.