If what you mean by adopt bitcoin as a state currency means replace their existing fiat currency; I believe there would a lot bigger problems to consider. If they did do that, it would essentially be saying that their money no longer has any value, which would cause a massive panic within their population. A type of panic similar to when India banned the 500 and 1000 Rupee notes.

You'd also have to consider countries that have that type of GDP, they would need the infrastructure for everyone to be able to be connected to the internet and have devices to use Bitcoin. Also they would need everyone to be tech savy enough to understand how to use bitcoin.

Also, with the lightning network at its current state, bitcoin transactions can't scale to an entire countries' economic activity.

So I would say that a 51% attack is pretty low concern for a country looking to adopt bitcoin as a state currency. A better approach would probably be to start stock piling bitcoin and perhaps even use it to back their fiat currency to strengthen it.

You would not do PoW if you have a state as central authority and trust anchor. You would simply switch to Proof of Authority or other strategies, e.g. permission-ed blockchains. Further, you could use other Decentralized Ledger Technologies.

PoW is so great because everyone can join the network without revealing their identity.