Need help with weapons / ascension please...

Well lightning is a standard way of making weapons deal high damage without high stat scaling, so people typically use those with smaller weapons. If you are interested in Int or Faith, both can provide buffs which turn light weapons into nightmares. The Balder Side Sword is a very decent small weapon with good range, fast attacks, and two types of damage, Slash/thrust. As well you can lead targets with your R2 of the weapon, and if upgraded normally it can be buffed to deal great damage in bursts. Chaos is another option, but you will need a +5 Fire weapon, and the Chaos Ember found near where the Ceaseless Discharge stands. Chaos Weapons outdamage lightning when you have 10 Humanity active, otherwise they'll deal less damage. With smaller weapons a lot of them don't scale too high, so in some ways Elementals are a bit better of an investment unless you are going for buffed weapons. As well that is always a way to increase AR if you aren't going for any type of magic, buying Charcoal Resin to flame but them.

In regards to titanite Chunks, in the Kiln you can farm every type of Chunk, there are Black Knights there, and each one carries a single type of chunk with a 100% drop rate. Slabs though for +15/+10/+5 max upgrade, those are harder to obtain and will require farming in certain areas. You can find what enemies drop them on the wiki.

I hope this helps.

I am playing a dex build atm and using quick weapons. I have a lightning Uchigatana+4, A Laito+10(going to +15) and a queelags furyswoord +4 and the Uchigatana and furysword kicks ***. I am about lvl 70 something aswell.

I am regretting that i did'nt make a washing pole lightning instead of the uchigatana tho =) But i can really recomend the katanas if u want quick attacks. Plus they have the bleed effect wich rocks on some bosses =)

What you look liek you need is a claymore. Its got a C in both str and dex so your scores will help scale the damage up nicely. Its good a good moveset too which is important if you're going into pvp. +15 with a buff will give you an amazing amount of damage. It is a bit of a heavy weapon though I believe its one of the faster ones.

The Undead Merchant you want to keep alive for invasions, he's the only source of Talismans. You can join the forest hunters and talk to Shiva by Alvina, then head to Btown and buy Katanas from him, including the uchigatana.

Buffing is adding an effect to your weapon. So, if you have charcoal resin and a +15 Claymore, you can use the Resin on it to add Fire Damage, if you have a spell like Sunlight Blade, you can cast the spell giving it a high damage lightning effect. The highest damage done possible with weapons is via Scaling and buffing.