Extender increases your HP and/or SP. Charms increase your STR, DEF, or both. And yes, they are cash-shop items.

I dunno about other fighters but the things that worry me the most in pvp would be mages and charmed-up, extended DD fighters. Mages because I can't even hit them if I'm dead before I get close (my mdef is hideously bad... it's like 4xx unscrolled) and DD fighters... well, they do a lot of dmg but are squishy unless they got charms, extenders, or +9 armors.

There are some fighters that are good against mages. Some carry around mage-killing sets that have high END and SPR. The trick is knowing how to fight them considering that DD fighters have really bad mdef generally. There are some tactics in 1v1 that I find useful but timing is hard and not every mage plays the same, so there's still quite a bit of room for error. Our high dmg output is no good to us if we can't get close enough to attack anyway.

I remember from my Diablo 2 days dueling another player meant you couldn't use anything to heal yourself, does this "unwritten" rule apply to fiesta pvp aswell?

You could heal yourself in duels, but generally, only stones and healing skills (cleric's heal and fighter's vampiric strike) are used. Healing yourself with potions is usually looked down upon.

Potters fail at life.

My dmg is2039-2333 and my def is 1000 is this good or bad?

It depends on whether or not you have buffs (pt, marriage, guild) and scrolls. Also depends on your gears.

Without pt buff without PoL with guildbuff

Erm~ Well, you also have to look at other players builds and skill distribution points. You cant say wether another fighter is weak unless they have the same build as you and fight the same way as you, some people have slow reaction times and other things that can seperate them from being a "good fighter" pvp wise I mean.

Like this: I have a two warriors iSpirit[Lvl 85] and Zanori[Lvl 89] , Zanori being my first fighter though. I made Zanori 25 Spr Rest End, no +9 gears until i capped him. iSpirit was made Full Str +9 gears up to this point, both have the same cooldowns meant for an axe fighter. I usually dual log (shh dont tell OS Dx) with Zan in vendor while i do repeats and other 80+ quests with Spirit, one day I decided to match them up and see how they'd fare against each other in pvp so i called my bestfriend over and let him pilot Zan from my laptop (already logged in) while i piloted Spirit and we pvp'd no extenders/no stones/ no cs gear/ no cs at all. Sppirit beat Zan 5-1 (Spirit being under my control) then we switched and I piloted Zan beating Spirit 5-2, same type of match. My friend plays the game also, we started the exact same time in the game and he has a capped End Str built fighter, so expertise was not an issue, I allowed him to change the skill shortcuts to suit his liking.

As to being overpowered.
Look at it this way, if they are using means that arent provided by the game (i.e hacks) they arent overpowered. They have access to almost everything you have, by this i mean, any character can find godlies, sell them, make golds and buy +9 gears and weapons. The only thing that seperates you from them is wether they have the means of using cash shop.