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

1. Coin Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in good condition. This generally translates to a higher price compared to coins with lower conditions.

2. Current Price and Previous Prices: The current starting price is 900 euros. In previous auctions, the average hammer price has been 302.33 euros, but there are also examples of significantly higher hammer prices, such as 1200 euros in a previous auction.

3. Price Trends: By observing the hammer prices of coins with similar conditions, we can see considerable variability. For example, in recent auctions, coins with conditions between 70 and 75 have reached hammer prices of up to 320 euros, while others with conditions of 50 have had lower hammer prices.

4. Demand and Supply: The coin has been auctioned 30 times, with a high sales percentage (28 out of 30). This suggests that there is consistent interest in this coin, which could increase its value in the current auction.

5. Comparison with Previous Prices: In the most recent auction, a coin with a condition of 70 sold for 320 euros. This suggests that, given the condition of the coin in question is higher, the hammer price could be considerably higher.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 1000 to 1200 euros, considering that the condition is good and there are precedents for high hammer prices in the past. However, since the starting price is 900 euros, bidders may be willing to pay a little more, but not much beyond this range, unless there is intense competition in the auction.