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

1. Coin Condition: The coin being auctioned has a condition rating of 100 (MS/PR), indicating that it is in excellent condition. This generally increases its value compared to coins with lower condition ratings.

2. Auction History:
- The highest recorded hammer price for a coin with the same condition (100) was 220 euros in December 2020.
- In other auctions, hammer prices for coins with similar conditions (95) have reached up to 60 euros.

3. Starting Price and Average Prices:
- The current starting price is 90 euros, which is significantly higher than the average starting price of 17.2 euros and the average hammer price of 35.5 euros. This suggests that the seller has high expectations for this auction.

4. Sales Trends: Of the 18 times it has been auctioned in the past, it has sold 15 times, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 180 to 220 euros, given that the highest recorded hammer price was 220 euros and considering that the condition is excellent. However, since the starting price is 90 euros, bidders may be willing to pay a bit more, but not necessarily reaching the historical maximum.

Therefore, a reasonable estimated maximum purchase price could be 200 euros.