I agree they wont do 15W certainly not whitelight and the power meter on the front of them only goes to 10W as far as I know. I got the impression these were local to you but now you talk of shipping - have you seen them have you seen them running? I would be very careful as they could be expensive bricks if they dont work or are at end of life - maybe a broken tube

. Especially if you say you cant really afford them - can you then afford for them not to work. With something like this I would insist to see them working and would collect in person even if it meant hiring a vehicle or making several trips. If the seller does not know how much power they are or cant power them up for you in preson then I would doubt their credibility.
However if all of this is wrong and you do buy them & they work fine, yes the power will be mega but not only that you will see colour range and depth like NO dpss/solid state can make. A gas whitelight is a thing of amazing beauty see some pix of my sp168 whitelight here -
http://www.stanwax.fotopic.net/c662316.html
I hope it all works out for you
Rob