You're referring to playing acoustic parts and sections found in some metal songs? In that case I'd recommend something with a deep tone, and downtune by maybe half a step to E flat for a darker tone, but of course it depends what key the song is in. There's no hard and fast rule on which acoustic you should get for this purpose but something with a bigger body, e.g. a dreadnought or jumbo, should do it fine. Many metal guitarists, when they record acoustic parts, also add in chorus and modulation effects as an enhancement so getting an acoustic with an in-built pickup would be useful.
I'd recommend either Jack and Danny guitars (some have Fishman pickups despite costing less than $400, Fishman pickups are supposed to be quite good, and they have a good reputation) and Cort guitars. Jack and Dannys can be found at Ranking Sports and Music as well as Music Theme. Corts can be found at Swee Lee outlets.
Hope this helps!
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