Interesting, as the rules prohibit you from using 1/3 of your skills. Doesn't sound fair to me, but I'll try to assist anyway.One question though, are you allowed to complete all the quests you want to get extra skill points? If so, you will have access to 61 skill points by 50. Of course, getting to and completing the quests in Hell will be tough by level 50, but it could be done. I'll go ahead and base this on having 57 skill points to work with (49 levels gained and 8 from normal and nightmare quests).
You'll need to decide on 1 method of dealing damage and 1 curse. Versatility is nice in PvM, but the streamlined approach is better for PvP. I'd say your options are 1) Poison Nova and Lower Resist, 2) Bone Spirit and Lower Resist (although I'm not 100% sure that Lower Resist still works with BS), 3) Bone Spirit and Decrepify, 4) Iron Maiden and Bone Shield.
1) Poison Nova IMO isn't good for duels. It is much too slow even with LR. Don't forget, in PvP, magic damage is reduced to 1/8. In the time it takes to do a significant amount of damage, you will be dead, assuming your opponent makes a good PvP Pally with Vigor.
2) A level 20+ Bone Spirit does fairly good damage and with fast cast, you can get a lot of them out there. If LR still works, you will be able to do a huge amount of damage. Of course with a high level vigor, the Pally can pretty much outrun the spirits and you are screwed.
3) Vigor is why you may want to use Decrep. That will keep him slow enough so that the Spirits can do there thing. Plus, weakening him is always nice because charge with might or fanat will do huge damage.
4) Iron Maiden and Bone Shield may just be the best way to go up agianst a Pally. Put all your stat points into vitality. That, combined with a level 20+ shield should allow you to survive the charge. I know, your probably thinking you should put points into strength and or energy also, but its not needed. First of all, strength only allows you to wear armor. To get the effect of IM, you need to get hit so you don't want to wear heavy armor with high DR. Instead, Leather or Quilted socketed with 2 Perfect Rubies should do the trick. Same with the helmet. You don't need any mana either since you should only have to cast a Shield and IM once. So, that should give you 255 points into vitality (counting 10 from the book quest) for a total of 510 life. Add in the life gained from the Statue quest and you are up to either 530 or 550. Now, in 49 levels, I believe you will gain 49 more life. You are now close to 600. You should be able to (assumig you can get your hands on them) wear 4 Perfect Rubies on your gear for another 152 life. So, not counting any charms or other equipment, you will be at 750ish life. I think a level 20 Shield will absorb another 220 life so you are almost at the 1k mark. Now, if I remember correctly, a level 20 Iron Maiden will give back about 600% damage? That means that in order to kill you, the pally will need to be able to absorb 6000 damage himself. Of course, that all goes out the window if the Pally is planning on using a bow (not likely) or Fist of the Heavens or possibly even a high level vengeance. You could very easily be screwed with this build. However, most PvP pallys go with charge and bonesnap (atleast the hardcore PKers do) and you would absolutely own him if he tried that. You might also want to consider a lower Bone Shield and adding in a mid-high level Bone Spirit just in case.