When I googled the Hiede Knight one of the first things that got me thinking was the tathered cloth he wears over his armor. I think it would be really cool to have a Knight with a sort of tathered robe.
As for the things sticking out I can only think of 2 ways to go for that look. One would be bullet holes (big ones, or holes where a melta weapon has melted a hole trough the hull) and slashing marks from certain melee weapons or giant creatures, or perhaps enemy knights. The other would be being partially corrupted by chaos, say one of the shoulders and part of the head or something, where a big tumor would be with horns growing out. This could also be reason for why he is the knight hunter maybe (chaos corruption, but maybe he is semi in control of it)?
As for the loadout one of the melee options is a must to me if his purpose is hunting the big bad. Maybe even both a sword and a fist? Or perhaps you could model him standing on something (which would increase the cost depending on what you want him to stand on), give him a shield and the sword, and count him as the forgeworld knight lancer (shield and spear), but you should ask your playing group if they would allow this count as conversion. (the standing on something is to increase the models height
btw, as the forgeworld knights are taller than the
GW ones)
If you dont want to go double melee or count as lancer,
Id say the melta cannon would make the second most sense for hunting enemy knights etc. But all in all for the ranged weapon you can pick the one you like most, they are all pretty badass. As for a carpace weapon, im not sure if that would fit the look, but the stormspear rocket pod can dish out some damage to the big baddies.
One of the best looking ways to show damaged armor plating is to dremel dents into it, sort of like this (
http://2.bp.blogspot.com/_YdjHwpj8nWw/S-NPb2ZZ3AI/AAAAAAAACR4/hpVmIUJo0Wo/s1600/IMG_0009.jpg) If a few of his armor plates looked like that, with feign hints of his old colors, that would make for an epic look I think. Also just painting really dirty mechanical parts (Leadbelcher with Typhus Corrison works like a charm for this, and maybe even some ryza rust on some parts, with highlights of runefang steel) and all pneumatic cylinders heavily washed with nuln oil etc. Make for an aged knight for sure.
Another thing you could do is have different armor plates or even limbs be in sort of different colors. As a sign of him replacing damaged parts with parts from knights he has slain etc. For example if you go with the dremel denting from earlier, you could have one armor plate be perfectly new (or maybe just a few scratches) in a totally different knight household color, that he just recently took.
I hope anything in this huge wall is of use

The concept of a knight made to hunt the others sounds really cool to me though, would love to see pictures of it over at the P&M blog section