I use an old minolta rokkor 50mm Macro reversed on bellows via an eos>md adapter.
the problems I have had are:
the whole set up is very heavy, so handheld is not an option
the viewfinder is very dark so precise focusing (critical with such shallow depths of field) is cumbersome (especially as I use a micro adjust rail)
you need to use manual exposure (not that much of a problem, but not as fast as using AV as you need to adjust to get it right)
The pros are:
This set up was quite cheap as I only had to buy the EOS adapter
The image quality is pretty good
the magnification ratio is very high as well (up to 5x)
However for the reasons I have stated I think that such a set up would be impractical for butterflies, the long exposures and cumbersome set up will just be too slow, and if you are relying on the screen outdoors to set a manual exposure then it could be tricky.
I agree with the advice to try your own kit as is, you want to have some background for context anyway, the EOS450 has a high enough resolution that you can crop a fair bit if required. I would degrade the optics with screw in close up filters.
If you are set on getting a macro lens proper then look for the sigma 50mm or 70mm, these crop up occasionally used.
They require a closer working distance but are optically very very good for the money. I tried the canon 50mm macro (which I used to have on my old film EOS5) on a 400 and wasn't impressed.