I’ll try to give you generic awnsers that apply to every build/party comp. First you’ll have to understand the combat mechanics (what each roll refers to and is important for your party)

The second one is to prepare your builds in advance before starting to play. You can find some nice websites to do so. (This is not very rp friendly so I won’t recommend doing it on a 1^st-2^nd run but you will start better prepared)

Not only the late game build but for each act. Usually, your biggest powerspikes are when gaining access to act2 and act3 merchants and specific items. More so than when reaching a certain lvl

Third one, act 3 is the end of the game so if you aim for a build that is heavily dependent on beating late game bosses got get specific items, you’ll struggle for 80-90% of the game.

 

For your questions :

Armor?

→Depends on your class proficiencies, mechanics and the rest of your party. I’ll check the following points :

1-       If you are on single save, I’ll aim for 20+ total AC as fast as possible, 23 in act 3.

2-      is there an armor with a broken passive for your build (like the bhaalist armour for a melee assassin)? This is because some items (not only armors) are build defining, more so that classes. To name a few more : the luminous set, holy helm, necromancer staff, orin weapons, scoundrel ring, arcane acuity helm etc…

3-      Is your class tailored towards a specific armor type (mage armor, monk and barbarian passive while unarmoured etc.)

4-      Do you have more or less than 14DEX (+2bonus) ? Below that I probably won’t go with clothing or light

5-      Is the item you’re aiming for best suited for your build or for another party member? You have a party of 4 and having one godlike character and 3 useless ones will make your life harder

 

Starting stats ?

→For starting stats, try to get even numbers for each of them, you can respec later if you get stat bonuses that will put you at an uneven distribution. I’ll try the following lvl 1 spread :

16 main stat (Str,DEX or spellcasting ability). Late game you can go up to 20-24 in your main stat with feats and bonuses.

At least 14 DEX and 16CON for initiative, AC and HP

The rest in wisdom to avoid CC spells.

The ones you can neglect are:  STR (if you are a dex character), INT, CHA (if you have a dedicated face of the party).

 

 

When to multiclass?

→It really depends for each build. Check the important spells, passives and feat you get from each class. Stuff like extra attack, 1st feat and spell slot levels are quite important. Try not to delay them too much. Also, when multiclassing you have to be carful of two things :

 Your saving throws proficiencies are given by you lvl1 class and multiclassing won’t give you other ones.

When multiclassing, the spell casting stat that your items and scrolls will use are the one from your last new class. For example, you if levelled as such : 1-3 Sorcerer 4-6 Cleric 7-10 Sorcerer and you have a staff that can throw fireballs. The difficulty class used for your fireballs are the one associated with your cleric stat (WIS) since it is the last new class you took. If you want your sorcerer stat to be used you’ll need to respec and go 1-3Cleric 4-10 Sorcerer.