7kg should be fine for a 15footer - as long as you've a decent length of chain as well
my personal preference is for a bruce type anchor, but keep a danforth on board as well. both 7.5kg with 10metres of 6mm chain on a 16 foot boat, gonna add another 10metres of chain soon
The bruce seems to hold better on rough/mixed ground but was dragging like buggery in the strong wind on sunday (hence gonna get some more chain) anchored off herd sands for the QE2 coming in, but I couldn't be bothered switching to the danforth
the length of rope depends how deep you plan to anchor - 3 times the depth as a general rule of thumb. I bought a 220metre roll of 12mm multiplait poly for 30 quid so stuffed the whole lot in the anchor locker. wouldn't want anything thinner than 10mm, not for the lack of strength, but it knacks to haul in smaller ropes