There is nothing wrong with your plan to reverse a manual focus/manual aperture lens. Especially since it sounds as if you already have the lens. Depending upon the lens you choose it should work well and simple, inexpensive EF mount extension tubes to use with it would be an option to increase magnification.
Regarding the flash... Yes the single 430EX II will work, but you will need a couple things:
1. Get an off-camera shoe cord. This allows you to handhold the flash just about anywhere around your subject. If the Canon OCSC is too pricey, there are third party avail. much cheaper that work just fine. After all, they are merely a bundle of wires and a couple connectors: one for the hot shoe, the other for the flash.
2. You'll need to diffuse the flash somehow. A cheap and easy way to do this is put a couple layers of white gauze bandage over the flash head and hold it in place with a rubber band. You can vary the amount of diffusion and strength of the flash by increasing or decreasing the number of layers of gauze. I usually use 3 layers on my 550EX or 580EX II flashes.
Between those two, you'll end up with something that looks like this...
You will need the flexibility to position the flash that this gives you. The flash can still be used in ETTL mode for automatic exposure and with the diffusion, depending upon how you position the flash and how small your subjects are, it will act like a giant soft box in the sky and you can get results like this...
... shot with 30D, Canon EF 100/2.8 USM macro lens, and 550EX flash w/gauze diffuser (handheld).
You also might want a flexible macro flash bracket... Something like this one
, for a single, large flash. Alternatively, there are dual flash brackets for macro work with smaller flashes, but you will need to figure out a way to control the two or use the flashes manually.