To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

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

2. Auction History:
- It has been auctioned 4 times in the past, of which 3 have sold. This suggests there is interest in the coin.
- Previous starting and hammer prices are relevant. For example:
- In 2018, the starting price was 400 euros, but the hammer price is unknown.
- In 2017, the starting price was 250 euros and it hammered at 380 euros.
- In 2014, the starting price was 400 euros and it hammered at 400 euros.

3. Average Starting and Hammer Price:
- The average starting price is 337.5 euros and the average hammer price is 370 euros. This indicates that, in general, coins of this reference tend to sell above their starting price.

4. Current Starting Price: The current starting price is 300 euros, which is lower than the average starting price and previous starting prices.

Based on these factors, we can estimate a maximum purchase price. Given that the condition is good and the interest in the coin seems solid, a hammer price that could be considered reasonable might be in the range of 400 to 450 euros, taking into account that the starting price is 300 euros and previous auctions have shown hammer prices in that range.

Estimated maximum purchase price: 450 euros.