Now that my pvm dexxter (macing) has almost hit the skill cap I wonder if I should upgrade my weapon and armor (as for now I'm wearing the cheap ones bought from the npc).
And if yes, what kind of weapon and armor should I buy?
100% should change your weapon and armor immediately, try player vendors around moongates, you can start with GM crafted exceptional armor, it's leaps and bound above NPC stuff, when you get comfortable with the thought of PK's killing you and taking your armor, feel free to upgrade it to better ore types, starting with lesser ones like shadow or agapite... Probably more important on the weapon, as it'll get you more accuracy, make you kill faster - while the higher AR helps, it's not really game altering mid level.
Also, pop by the discord for Outlands, you'll generally get far quicker response in the #newplayer channel.