Build all three of them.

Kael is good for early clan boss teams.

Skullcrown is good for brutal 12-3 farmer and good for arena. 

Dhukk is good for dungeons as offensive support and can be use in arena too.

Reliquary Tender is a good early game reviver if you need one.

Dark Elhain is also a good damage dealer in arena and also good in Ice Golem.

Coldheart is good in fire knight and spider dungeon, but I suggest not to build her at early game as she is hard to keep alive if you do not have good gears.

Keep Psylar for now, while she can be useful at end game contents like Faction Wars and Doom Tower, she has very little use in early game accounts.

Shatterbones is an underated champion for arena. I have not build him yet but his multipliers looks pretty decent as arena nuker.

so  would  it  be  better  to  book  dhukk  since  dungeons  are  more  important  than  arenas  (i  think)  becuase  i  have  9  epic  books  right  now .  also  which  5  do  you  think  should  make  up  my  dragon  team

Easy: SkullCrown to 60 now. Is The Queen in Arena and farmer campaign in 7 seconds. But yeah....in your Level...Kael can help you in Clan Boss with poisons (poisons are good mode of damage in Clan Boss) and every place in the game

If I were you I wouldn't stress about using any books yet, because at your current level the champions will do fine unbooked. 

Your goal right now is to get a level 60, and since Kael is already 50 I would push on with him.  You need to get to stage 12-3 in campaign on Brutal difficulty in order to have an effective food farming platform (edit, by this I mean the ability to run a single lvl 60 champion with 3 low level champs that will be used as food for maximum experience).  

The next step after that is to push on with Dragon as much as you can, as well as Clan Boss.  Kael will be the backbone in all 3 of these areas.  Kael will also serve as a nuker in Arena. Only then do you have to start worrying leveling up a second champion to 60 who might be more niche

I would book Dhukk to at least max his a2. After that, I would not bother maxing his other skills. 

You will end up with a lot of maxed champs in this game. Basically, it's what the game is about. Max out different champs, to have different teams that can beat every content of the game.

The question is not, if you want to max Kael or Dhukk or Skullcrown, the question is, who should be maxed first.

Some champs are bad and useless, exept that they can serve as food in the tavern. But be carefull before you sacrifice champs. Some of them seem pretty useless at the beginning, but are very powerfull in certain niche strategys in endgame content. For example, you have the rare dwarf Master Butcher. Don't care about that champ now, he doesn't help an early account in the content you are doing now. But definetly don't use him as food, keep that champ in your vault. He is great to fight the Doomtower boss Bommal.

You should activate this symbol at all champs JoinME mentioned, and on Master Butcher and Bogwalker (rare Skinwalkers champ), to prevent that you use them as food accidentaly.

i

Also, keep all champs with the following symbol at least once, they are fusion champs.

i

 I look at it in order of priorities. The first thing to do is get a champion who can solo farm 12:3 brutal in Campaign since that's where most of your XP for leveling up other champions will come from. The next thing you need to do is get to nightmare level Clan Boss so you can start winning shards and books. To do that you will need 5* and 6* Speed and Lifesteal gear from the Dragon dungeon. After that you can starting looking seriously at Arena. Kael is good in all those areas and you've already got him 5*, I'd finish him out and work on building a Clan Boss team.