The danger in using a soda syphon, and second hand soda syphons especially is that they are usually made of glass which can shatter under the pressure of the nitrous oxide released into it. It is much safer to buy a whipped cream dispenser as these are made of metal and there is no danger of exploding glass near your face when you are using it. The best way to clean them is to hand wash the inside of them with dish washing detergent like you would normal dishes, dry inside and out with a tea-towel, and then lubricate the rubber o-ring seal with vaseline to prevent it from becoming dry and cracking. This will provide a tight seal that will not let the nitrous escape. You can buy used whipped cream dispensers from op shops for as little as $10, just make sure all the parts are there and the o-ring is in good condition. It is an unnecessary risk to use a glass syphon.