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

1. Current Condition: The coin has a condition of 80 (MBC+), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: By looking at previous auction prices, we can see that:
- In 2020, it was auctioned for 260 euros with a condition of 70.
- In 2016, it was auctioned for 295 euros with a condition of 85.
- In 2015, it was auctioned for 340 euros, although the condition is not specified.

3. Starting Price and Average Price: The current starting price is 350 euros, which is higher than the average starting price of 257.27 euros and the average auction price of 313.3 euros. This suggests that the seller has high expectations for this auction.

4. Sales Trends: The coin has been auctioned 11 times, of which it has sold 10 times. This indicates good demand and a history of successful sales.

5. Comparison with Recent Auction Prices: In the most recent auctions, the hammer prices have been in the range of 380 to 400 euros for similar conditions (80 and 70).

Based on these factors, an estimated maximum purchase price could be in the range of 380 to 420 euros, considering that the condition is good and there is a trend of increasing prices in recent auctions. Therefore, a reasonable maximum purchase price could be 400 euros.