To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data, especially previous auction prices and the context of the current auction.

1. Current starting price: 120 euros.
2. Auction price history:
- The highest auction price in the past was 320 euros (2015).
- Other relevant auction prices include 186 euros (2017), 110 euros (2020), and 100 euros (2021).
3. Average auction price: 55.6 euros, but this may be influenced by coins in worse condition.
4. Condition: There is no condition information for the current coin, but in the past, some coins of the same reference have reached high prices with conditions ranging from 80 to 95.

Given that the starting price is 120 euros and considering that significantly higher auction prices have been reached in the past, an estimated maximum purchase price could be in the range of 150 to 200 euros, depending on the competition in the auction and the interest of collectors.

Maximum purchase estimate: 200 euros.