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

1. Starting Price: 350 euros.
2. Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape.
3. Auction History:
- It has been auctioned 4 times in the past, of which it has sold 3 times.
- The average hammer price in previous auctions is 383.33 euros.
- The previous hammer prices are 340 euros (2020), 320 euros (2014), and the last hammer price is not available, but the starting price was 250 euros with an estimated price of 400 euros.

4. Price Trends: The average starting price in previous auctions is 281.25 euros, but the average hammer price is higher, suggesting that buyers are willing to pay more than the starting price.

Given that the condition of the coin is good (80), and considering the hammer price history, an estimated maximum purchase price could be set in a range above the previous average hammer price, while also taking into account the current starting price.

Estimated Maximum Purchase Price:
We could consider a maximum purchase price around 400-450 euros, given that the highest hammer price in the past was 340 euros and the estimated price for a future auction is 400 euros. This reflects both the good condition of the coin and the price trends in previous auctions.

Therefore, a reasonable maximum purchase price could be approximately 425 euros.