Hi
I am thinking of buying this kind of welding magnets like these
http://www.ebay.com/itm/4x-50Lb-Magn...8AAOSwXshWqqCr

As I have not use such tool before, I would like to hear your opinion about what to be careful of.
#1
It says 9lbs, 25lbs, 50lbs, 75lbs. Any opinion about how many lbs I will need for example for 40X40 80X80 rectangular tubes, just to have an idea about how strong magnets I will need for my constructions

#2
Other than ebay do you have any supplier that will ship international


Thanks for your time