To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition of 80 (MS+), indicating that it is in fairly good shape, but not the best possible condition.

2. Historical Prices: Looking at previous auction prices for coins with similar conditions:
- In 2020, a coin with a condition of 80 sold for 300 euros.
- In 2021, another coin with a condition of 80 sold for 340 euros.
- In 2022, a coin with a condition of 80 sold for 300 euros.
- In 2023, a coin with a condition of 85 sold for 360 euros.

3. Starting Price and Average Hammer Price: The current starting price is 350 euros, and the average hammer price for this coin is 307.38 euros.

4. Increasing Trends: In recent auctions, hammer prices have shown a trend of increasing, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price could be in the range of 360 to 400 euros, considering that the condition is good and there is an upward trend in hammer prices.

Therefore, a reasonable maximum purchase price could be 400 euros.