Learn Croatian: Neću nositi ovakav kaput jer je previše moderan. - I will not wear this kind of coat because it is too modern.

Ovaj means 'this' specific one (like 'this exact coat'). Ovakav is a demonstrative pronoun of quality, meaning 'this kind of' or 'such a'. The speaker is not just rejecting one specific coat, but any coat of this style.

What case is ovakav kaput, and why doesn't the pronoun have an ending like -og?

It is in the accusative case because it is the direct object of the verb nositi. However, kaput is an inanimate masculine noun (a thing, not a living being). For inanimate masculine nouns, the accusative form is identical to the nominative, so ovakav does not get a special ending here.

Why is the adjective moderan in the nominative case if the coat was in the accusative case earlier?

The word jer (because) starts a brand new clause. In this second half of the sentence, the coat is now the hidden subject (on je previše moderan), so the adjective describing it switches to the nominative case.

I usually see previše translated as 'too much'. Can it also just mean 'too'?

Yes! When used with a noun, previše means 'too much' (like previše vremena for 'too much time'). But when you place it directly in front of an adjective like moderan, it translates to 'too' (too modern).