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

1. Current Condition: The coin has a condition rating of 85 (MS+/EF-), indicating that it is in relatively good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for similar coins has been 257.93 euros.
- In previous auctions, coins with a similar condition (85) have reached hammer prices that vary, but some examples are:
- 360 euros (auction on 2020-11-24).
- 170 euros (auction on 2020-10-21).
- 150 euros (auction on 2020-12-16).

3. Starting Price and Estimated Price:
- The starting price is 150 euros, and the estimated price is 300 euros. This suggests that there are expectations for the coin to sell at a significantly higher price than the starting price.

4. Recent Trends: In the most recent auctions, hammer prices have shown a trend of being higher, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price could be in the range of 300 to 400 euros, considering that the average hammer price has been 257.93 euros and that there is potential for the coin to reach higher prices due to its good condition and market interest.

Therefore, a reasonable maximum purchase price could be 350 euros.