Just buy a BMX freewheel. You can get Dicta (cheapies) from most shops for $10-$20 and they're fine for what you're asking it to do. The above tool only works up to about 18 teeth, after that, you can have fun getting the sods off again, but don't let that stop you using bigger freewheels, I can usually get them off using a centre punch and if that fails, you can use a destructive method (they're cheap remember).

Chainline? Just spin on the freewheel, a wonky chainline won't bother you (remember how the chain has to flex with gears) but if you feel like mucking about, just mess about with spacers to get the chain line right, then redish the wheel so the rim is in the centre of the fork. You can do this on the bike and the spokes will have enough room on the threads to let you do it (Sheldon Brown has an article called 'Fixed gear on the cheap' or something similar that describes it).