actually if money is not a problem, getting a dedicated macro lens is the best option....
i bought the sigma APO for its range and also for macro... but found that its macro capabilities is not really there....
depending on the sizes of your cartoon figures, the sigma I think most probably will meet your requirements. Most wingspans of butterflies I shoot are about 3-5 cm big.
My setup is just a D70 and sigma 70-300 APO II... sometimes with flash sometimes w/o.
If it doesn't focus properly, there are many reasons. you have to understand how your AF system works. Make sure also you are at least further than the minimum focusing distance. This lens has a min dist of 95cm in macro mode.
a dedicated macro lens is definitely recommended if you can afford it. For me, I'm just happy with my current setup. If you want really close up macros, you can tyr a reverse 50mm or just use the 50mm. It has a lightly better repro of 1.8 compared to 1:2 for the 70-300
I myself use the same lens, sigma 70-300 APO II, with 300D.
It serves me well.
the problem of focusing, can you elaborate further? did you get too close? like yewkay's pointed out, min focusing dist is .95m
If you need to get closer (in order to get larger image), you can always stack up a close-up filter. you can buy canon 500D or nikon 5T or 6T.
if you have issue about sharpness, try using tripod or monopod. handholding at 300mm is quite challenging, especially indoors; assuming you're shooting ur figurines indoor.