To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the current starting price, the current condition, and the hammer prices from previous auctions.

1. Current starting price: 900 euros.
2. Current condition: 80 (MBC+).
3. Average hammer price in the past: 1185.71 euros.
4. Previous hammer prices for similar condition (80):
- 950 euros (2018-10-17)
- 2400 euros (2018-05-31, but with condition 95)
- 800 euros (2017-04-26, but with unspecified condition)
- 1200 euros (2022-02-17, condition 80)
- 950 euros (2023-03-16, condition 80)

5. Price trends:
- The coin has had varying hammer prices, but generally, the hammer prices for similar conditions (80) have been around 950 to 1200 euros.
- The average hammer price is 1185.71 euros, suggesting that buyers are willing to pay close to this value.

6. Estimation:
- Given that the condition is 80 and considering the previous hammer prices, a reasonable maximum purchase price could be in the range of 1000 to 1200 euros, as this aligns with previous hammer prices and the average price.

Therefore, an estimated maximum purchase price for this coin could be approximately 1100 euros. This allows for a margin for competition in the auction, given that the starting price is 900 euros.