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

1. Current starting price: 900 euros.
2. Coin condition: 80 (MBC+), indicating good preservation.
3. Auction history:
- It has been auctioned 6 times in the past, of which it has sold 4 times.
- The average starting price in previous auctions was 641.67 euros, and the average hammer price was 752.5 euros.
- Previous hammer prices varied, but the highest was 750 euros in 2014.

4. Previous hammer prices:
- 2018: Not sold (starting price 900, estimated 1500).
- 2014: Hammer price 750 (starting price 600, estimated 1000).
- 2013: Hammer price 460 (starting price 450, estimated 750).
- 2022: Hammer price 700 (starting price 600, estimated 1000).
- 2022: Not sold (starting price 400, estimated 650).

Given that the coin is in good condition (80) and considering the current starting price of 900 euros, which is higher than previous starting and hammer prices, it is reasonable to assume that interest in this coin could be greater in this auction.

However, it is also important to note that the average hammer price in the past has been lower than the current starting price. Therefore, an estimated maximum purchase price could be set in a range that reflects both the good condition and the price history.

Estimated maximum purchase price: It could be reasonable to consider a hammer price around 800-900 euros, given that the starting price is 900 euros and the hammer price history suggests that buyers might be willing to pay a little more, but not necessarily the full starting price.

Thus, an estimated maximum purchase price could be 850 euros.