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

1. Current condition: The coin has a condition of 50 (fine), indicating an intermediate state. This can influence its value, as coins in better condition tend to reach higher prices.

2. Auction history:
- In the April 2022 auction, a coin with a condition of 30 sold for 1200 euros, with a starting price of 300 euros.
- In the March 2023 auction, a coin with a condition of 85 sold for 1600 euros, with a starting price of 1000 euros.

3. Average hammer price: The average hammer price for this coin has been 1400 euros, suggesting that, in general, buyers are willing to pay around this amount.

4. Current starting price: The starting price is 900 euros, which may indicate the seller's expectation of the coin's value.

Given that the current condition of the coin is better than that of the 2022 auction but lower than that of 2023, and considering the average hammer price and the starting price, an estimated maximum purchase price could be positioned in a range between the average hammer price and the hammer price of the 2023 auction.

Therefore, an estimated maximum purchase price could be around 1400 to 1600 euros. This is based on the sales history and the current condition of the coin.