What gear would you recommend me to upgrade/buy next?

That monsterbook ring

 

Attack gloves and work on the monster book ring. Then when level 100 upgrade overall to Battle Road and shoes to Blue/Green crescent shoes

 

It would be helpful if you let us know what your budget is.
Also, there is a gear guide

You definitely want:
- 10 att wgs
- shoes scrolled for dex
- sad mask scrolled for dex
- higher tier monster book ring
- 3/4 att pink gaia cape

 

While I largely agree with Floris regarding upgrades you can get now, personally i feel you are in a decent spot, and anything higher (other than monster book ring) is kind of a privilege.

I'll personally suggest you to keep the mesos first, get a decent pole arm or spear for lvl 80-120/135 (not too sure on dk weap progressions) then get the best sky ski or pinaka/dragon faltizen? (unsure spelling) that you can afford to head start bossing

 

The best upgrades are the ones that need to be made the least often. Attack is pretty mediocre for improving exp/hr in 3rd job generally. DPS is most important when breaking barriers between 3ko and 2ko, or 2ko to 1ko, like at CDs. Even going from 3ko to 50% 2ko isn't that ideal because it's going to take an extra cast to finish regardless. Walking speed is better for the most part, with reducing time between moving to or gathering mobs exp improves. Most work is put in more from skill values, and DK 3rd job has great skill values already.

Conventional advice is to just stack more damage, but I don't actually believe that makes for large improvements for the associated cost. First upgrade I'd recommend is 3 pets with speed equips. 140 speed would make the largest difference. Kenta's quest is ideal too so they can loot quicker.

At level 100 you can purchase a Crimson Arcglaive and drop 60% scrolls on it. If summer event drops soon it's also possible to purchase a Purple Surfboard(Mostly depends on how they're acquired I suppose), preferably a cheaper one. Either one of the pole-arms is a good upgrade because it's a rather large DPS bump for a rather low cost. You can also buy clean Blue Crescent Boots but those don't need to be scrolled yet, or they can be scrolled for speed if needed. It's a smaller upgrade but it's cheap enough to be efficient.

12 dex cape can be replaced with a 3 att pink one, preferably one that already has no slots with some dex or str, they're generally cheaper than they are clean.

At 110 you can purchase a MoN from CWKPQ runners to replace the Silver Deputy Star. I'd aim for a minimum of 4 attack, with 5 being max.

The majority of mesos should be saved for purchasing decent gloves and decent Sky Skis. Gloves likely come first, I'd aim for 12 attack(Owling for odd gloves like Green Mittens or generic Work Gloves can save some money, as people put them for cheaper) with Skis being a concern at level 125+.