Jag vet inte vilken sort hon vill ha, så jag köper samma som förra gången.

Because sort is an en-word: en sort.
With en-words, Swedish uses vilken.

Compare:

• vilken sort
• vilken bok
• vilket hus
• vilka äpplen

So the choice of vilken is just agreement with the noun’s gender and number.

Here sort means kind, type, or variety.

So vilken sort means something like which kind or which type.

It is often used for things like:

• food
• products
• flowers
• materials

In everyday speech, sort and typ can sometimes overlap, but sort often sounds more natural when talking about varieties of a thing.

In this sentence, vill ha means wants to have / wants to get / wants.

English often lets want stand alone:

• She wants the red one

But Swedish usually needs the extra verb ha when talking about wanting an object:

• hon vill ha den röda

So hon vill ha is a very common pattern.

Because vilja is a modal verb, and modal verbs in Swedish normally take the infinitive without att.

So:

• hon vill ha
• jag kan komma
• vi måste gå

Not:

• hon vill att ha

This is similar to English she wants to have only in meaning, not in structure. Swedish does not use att after modal verbs like vill.

Because this part is a subordinate clause.

In Swedish subordinate clauses, the subject usually comes before the finite verb:

• hon vill ha

That is different from main clauses, where Swedish follows the verb-second rule.

So compare:

• Main clause: Hon vill ha den
• Subordinate clause: ...vilken sort hon vill ha

If there were a negation, you would clearly see the subordinate clause pattern:

• ...vilken sort hon inte vill ha

Because in a main clause, Swedish normally puts the finite verb before inte.

So the normal order is:

• subject + verb + inte

That gives:

• Jag vet inte

Not:

• Jag inte vet

This is a very important Swedish pattern:

• Jag kommer inte
• Hon gillar inte kaffe
• Vi förstår inte

Here så works like a conjunction meaning so or therefore, linking two main clauses:

• Jag vet inte ..., så jag köper ...

When så is used this way, the next clause keeps normal main-clause order:

• jag köper

So there is no inversion here.

Compare that with a fronted adverb like därför, where inversion does happen:

• Därför köper jag samma sort

So:

• ..., så jag köper ... = conjunction
• Därför köper jag ... = adverb first, so verb-second word order follows

The noun is left out because it is understood from the context.

samma means the same, and Swedish often omits the noun when it is obvious:

• jag köper samma

This means I buy the same one / the same kind / the same thing.

You could also say the fuller version:

• jag köper samma sort som förra gången

But leaving out the noun sounds very natural when everyone already knows what is being talked about.

Also, som after samma is the normal word for as in this pattern:

• samma som förra gången = the same as last time

Förra means previous or last in the sense of the one before now.

So:

• förra gången = last time / the previous time

This is different from sista, which means last in the sense of final.

Compare:

• förra veckan = last week
• sista veckan = the final week

So in this sentence, förra gången means the previous occasion, not the final one.

Also, after förra, Swedish normally uses the definite form of the noun:

• förra gången
• förra veckan
• förra året

Because Swedish often uses the present tense for a future action when the context makes the future meaning clear.

So jag köper samma som förra gången can naturally mean:

• I’m buying the same as last time
• I’ll buy the same as last time

Swedish does this very often. The time reference comes from the situation, not only from the verb form.

If you wanted to make the future more explicit, you could say something like:

• så ska jag köpa samma som förra gången

But the present tense is completely normal here.