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

1. Condition: The coin has a condition rating of 85 (MBC+/EBC-), indicating that it is in quite good shape. This generally translates to a higher value compared to coins in lower condition.

2. Auction History:
- The average hammer price for this coin in the past is 318.75 euros.
- In the 2015 auction, the coin sold for 900 euros, which is an exceptionally high price, but it also indicates that there is a market willing to pay a lot for coins in good condition.
- In other recent auctions, hammer prices have varied, but they have been in a range that suggests the coin has significant value.

3. Starting Price: The current starting price is 300 euros, which is a good starting point for the auction.

4. Sales Trends: The coin has been auctioned 9 times, of which 5 resulted in sales. This suggests that there is interest in the coin, although there is also a considerable number of auctions that did not result in sales.

5. Previous Prices: Previous hammer prices, especially the 900 euros, suggest that there is potential for a high hammer price, although this may depend on demand in the current auction.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 400 to 600 euros. This is based on the price history, current condition, and interest in the coin. However, given the history of high prices, a maximum of 600 euros could be reasonable if there is good competition in the auction.