A Paladin is a fun class to play (like the rest can be) however being the support class that you need to be can be difficult to pull off. @killerduki WAS probally one who knew more about Paladin Vanilla tanking then most (even me), may the light rest his characters soul.
Paladin are one fo the best healer in the game, even at Tier 3 they really start to shine. Priests, Druids, & Shamans are also great healers. they each have their respectable abilities, paladin are known for their fastest healing abilities.
11 into Holy for the AOE is always a must (in my book) as for the 20/31/0 Protection build, perhaps show me what you mean.
Any low level dungeon you can solo. Talent build wont matter. Now solo a dungeon as prot at the same level as you wont be possible.
As for gear I always had 3 sets (and never had space for anything else) one for healing, one for tanking, one for DPS. these sets stayed in my bank however unless I needed them.
The idea of ret is great for solo questing, so if you want to go ret enjoy it! its part of the leveling process!
PvP is useful, on PvP servers. I used to be on Anathema, and trust me, they were useful! Now that I am on Darrowshire, the need is obselete.
Engineering is a great profession to have as a paladin for both pvp or pve.
I have done discussions over this many times before... so you'll have to find those yon the forums.
The short version is My Paladin is my main. I am BS/ENG on my main, my Warrior (alt) has mining, while my druid (alt) is alch/Herb.
Considering I have one of every class, I have pretty much every profession I level with my alts, but I gave ya a quick sum of how I run my Pal/War/Dru since you have them too.
I hope I was able to break is down and help illuminate your questions. I will add I have written a tankadin gear guide, but I play mostly holy. if your looking to casually play and tank 5 mans, while healing for raids.... Try my 30/21/0 Talent Build