With the announcement of Riviera Line I would like to suggest the 175 DMU as these will be entering service late 2025 early 2026 and these will be replacing the HST sets left in Cornwall. So far they are due to operate between Penzance and Exeter and would be perfect for this route.
Another add we could see the Riviera is a 158 as they do occasionally run along this route. As this reply is being written there's a 158 out and about between Paignton and Exmouth