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

1. Current Condition: The coin has a condition rating of 80 (MS+), indicating that it is in relatively good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for coins of similar condition (80) has been 200 euros in a previous auction (2020-05-26).
- In other auctions, hammer prices have varied, but the range has been from 51 to 420 euros, depending on condition and demand.

3. Starting Price and Average Hammer Price:
- The current starting price is 90 euros, which is lower than the average hammer price of 170.22 euros.
- The average starting price of 102 euros also suggests that the current starting price is competitive.

4. Recent Trends: In the most recent auctions, hammer prices have shown an increase, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price could be in the range of 200 to 250 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate. However, since the starting price is 90 euros, it could be a good opportunity to acquire it at a lower price than its market value.

Therefore, a reasonable maximum purchase price could be 250 euros.