Indonesian - Apa(kah), -nya and others.?

Yes, '-kah' makes question words sound more formally. But it doesn't mean it is necessary to use 'kah' for formal usage.

The 'imbuhan '-nya', is not to be used in that context. '-nya' refers to his/her/their/its, etc. when used to indicate possesion, such as:

-Namanya = His/her/its name

-Rumahnya = His/her/its house

-Temannya = His/her/its friend

You might have confused the example you provided here.

*Instead of:

-Ketikanya kesana, dia membeli bir.

*Perhaps you meant:

-Ketika ia ke sana, dia membeli bir.

If this is what you mean, than this is perfectly correct.

However, informal speech would make such usage of '-nya' possible.

Quantifying is tricky in Indonesian. The formal and (of course) more acceptable is to put the quantity before the noun.

Using the noun before the quantity is also (but less acceptable) correct. Only in certain situations it would work. Such as ordering in a restaurant.

Situation 1 :You're going to buy 3 beers in a bar. Bartender came to you.

Ex 1:

BT: Are you ready to order, sir?

You: Yes. Beer. Three.

(Notice that the noun and the quantity is not in the same sentence. Making it more informal.)

Ex 2:

BT: Are you ready to order, sir?

You: Yes, three beers.

(This is more--although not generally use in this situations--polite and formal.

When quantifying, It's best to use the titling for the said noun.

Such as:

-Tiga botol bir. Three bottle beer. (Means: Three bottles of beer, because beers are usually bottled, or canned, or in pitchers)

-Tiga ekor kucing. Three tail cat. (Cats have tails, so that's how we refer to the cat)

-Tiga pasang sepatu. Three pair shoe. (Shoes come in pairs, the noun doesn't change if the quantity changes, so you need to use pairing title if the nouns you use are supposed to come in pairs)

-Tiga orang perempuan. Three human female. (Means: Three women. We use the title 'orang' to quantify humans.

Sorry for the incomprehensive answer, how this will help you somehow.