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

1. Condition: The coin to be auctioned has a condition of 100 (MS), indicating that it is in excellent shape. This generally increases its value compared to coins with lower conditions.

2. Previous auction prices:
- In 2014, a coin with a condition of 100 was auctioned with a starting price of 25,000 euros and sold for 36,000 euros.
- In 2019, another coin with a starting price of 25,000 euros sold for 28,000 euros, although the condition is not specified.
- In 2021, a coin with a condition of 85 was estimated at 30,000 euros and sold for 15,000 euros, indicating that condition significantly affects the final price.

3. Current starting price: The starting price is 25,000 euros, which is consistent with previous starting prices for coins of similar condition.

4. Average hammer price: The average hammer price for this coin in previous auctions is 18,600 euros, which can serve as a reference for the expected hammer price.

5. Market trends: Given that the coin has a high condition and has had a history of relatively high hammer prices, it is reasonable to assume that collector interest could lead to a hammer price above average.

Taking all these factors into account, an estimated maximum purchase price could be between 30,000 and 36,000 euros, given the auction history and the high condition of the coin. However, it is important to remember that the final price will depend on demand at the auction and competition among bidders.