To estimate a maximum purchase price for the coin that will be auctioned, let's consider several factors:

1. Condition: The coin has a condition of 100 (MS), indicating that it is in excellent shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction history:
- The average hammer price for coins of similar condition (100) has been 27 euros.
- In previous auctions, the highest hammer price for a coin with a condition of 100 was 30 euros.

3. Starting price: The starting price is 18 euros, which is relatively low compared to previous hammer prices.

4. Sales trends: The coin has been sold in 48 out of 63 auctions, indicating good demand.

5. Previous estimated prices: In past auctions, the estimated price for coins in condition 100 has been up to 30 euros.

Taking all these factors into account, a reasonable estimated maximum purchase price could be between 30 and 35 euros, given that the condition is excellent and there is a history of hammer prices supporting this figure. However, since the starting price is 18 euros, some bidders may try to acquire it for a lower price, which could influence the final outcome of the auction.

Therefore, a reasonable maximum purchase price could be 30 euros.