Generally, the complete front end (wheel, axle, complete brake system, forks, fork guards, fork guides, and triple clamps). You will need to get a steering stem machined as well, as your stem won't work. That is the toughest part of getting it to "just bolt on". Newer is better on the forks. MX forks come with MX valving, so keep in mind that the valving may not suit you. Don't forget that used forks usually need a rebuild as well.
Great catch on the geometry! There are way too many choppered out KDX's running around already. Some people don't think of this, or don't care / know the difference. They can be mounted higher in the clamps to a point, but to get them to mount normally AND not throw off the geometry, travel limiting spacers would be needed. There is only so much space before the front tire hits the front fender during full compression.
Oh, I forgot to reply about the bottom out bumper in my previous post. It can only be replaced by disassembling the shock. The cost for the bumper is about $20, but getting it changed is the hard (and more costly) part.