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

1. Condition: The coin has a condition rating of 80 (MS+), which is an acceptable level, but not the highest. This may influence the price, as coins in better condition tend to fetch higher prices.

2. Auction History:
- The average hammer price for this coin in the past is 1147.33 euros.
- Previous hammer prices for coins with a similar condition (80) varied, but there are examples of auctions reaching up to 1700 euros (in the case of the March 2022 auction).

3. Starting Price: The current starting price is 900 euros, indicating that the seller has reasonable price expectations, but not necessarily the highest.

4. Price Trends: Observing previous hammer prices, it seems there is a trend for coins with a condition of 80 to sell in a range of 800 to 1700 euros, depending on demand and other market factors at the time of the auction.

Based on these points, an estimated maximum purchase price could be set above the average hammer price, but considering the condition and the starting price, a reasonable maximum price could be:

Estimated maximum purchase price: 1300 euros.

This price reflects a balance between the price history, the condition of the coin, and the current market context.