I second ufo's suggestion.
The PR Mclux will handle the heat alot better, and the Pelican reflector (supplied) will be alot nicer with the 5W.
If you are going to only use 2x123 batts, bo with a BB converter. But, if you wish to use 3x123, or something else down the road, you might want to look into the DownBoy converters.
700mA should make a very nive amount of light from the 5W.
But, I think the PR head could handle upwards of 1000mA.
You just need to deside what kind of brightness / runtime you want.
If you get the PR mclux head,, all you need is epoxy, 5W LS, converter, and batt pack.
If you go with that original Mclux haed, you will also need an ecan and nxo5, or Fraen LP optics.
I wouldnt drive the original Mclux head over 700mA though.
[EDIT] Let me clear something up. -- You donnot NEED an ecan for the original Mclux head. It is simply an option.
There is some debate on weather it is better to use the ecan or not. Some say that for thermal propertied of the converter, it is actually better not to.