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

1. Current starting price: 900 euros.
2. Current grade: 80 (MS+), indicating good preservation.
3. Auction history:
- In 2020, the coin was auctioned with a grade of 75, with a starting price of 600 euros and a hammer price of 850 euros.
- In 2019, the coin was auctioned with a grade of 85, with a starting price of 1100 euros and a hammer price of 1700 euros.

4. Average prices:
- Average starting price: 866.67 euros.
- Average hammer price: 1516.67 euros.

Given that the current grade of the coin (80) is better than that of the 2020 auction (75) and slightly lower than that of 2019 (85), we can infer that the hammer price could be in an intermediate range between the previous hammer prices.

### Maximum purchase price estimation:
- Considering the highest hammer price (1700 euros) and the lowest hammer price (850 euros), we can calculate a range:
- A reasonable hammer price for a grade of 80 could be between 1000 and 1300 euros, as it is better than that of 2020 but not as high as that of 2019.

### Conclusion:
An estimated maximum purchase price for this coin could be around 1200 euros. This is based on the grade, price history, and market context.