To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the condition of the auctioned coins.

1. Current starting price: 900 euros.
2. Average auction price in the past: 1195.45 euros.
3. Auction price history:
- 2018-05-31: 950 euros (condition 90)
- 2018-03-08: 950 euros (condition 80)
- 2016-10-19: 800 euros (condition 90)
- 2020-06-24: 1650 euros (condition unknown)
- 2017-06-28: 950 euros (condition unknown)
- 2021-03-09: 1300 euros (condition 95)
- 2021-02-18: 1300 euros (condition 80)
- 2022-03-30: 1700 euros (condition 90)
- 2022-09-27: 1300 euros (condition 90)
- 2023-02-16: 650 euros (condition 80)

4. Price analysis:
- Auction prices have varied between 650 and 1700 euros, with an average of 1195.45 euros.
- The conditions of the auctioned coins have influenced the prices, where higher conditions (90 and 95) tend to have higher auction prices.

5. Estimation:
- Since there is no information about the condition of the current coin, it is prudent to consider an auction price that is in the mid-range of previous prices, while also taking into account that the starting price is 900 euros.
- A reasonable auction price could be between 1200 and 1500 euros, based on the price history and the average auction price.

Estimated maximum purchase price: 1500 euros.

This price reflects a balance between the auction price history and the uncertainty regarding the condition of the current coin.