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

1. Condition of the coin: The coin to be auctioned has a condition rating of 90 (EBC), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Starting price: The starting price is 600 euros. This is the baseline for the auction.

3. Price history: By looking at previous auction prices for similar coins in comparable condition, we can see that:
- In past auctions, coins with a condition rating of 90 have reached hammer prices of up to 495 euros (in 2017) and 400 euros (in 2020).
- The overall trend shows that coins with similar conditions have had hammer prices ranging from 300 to 600 euros.

4. Average hammer price: The average hammer price for similar coins is 342.18 euros, suggesting that the hammer price may be below the starting price.

5. Auction expectations: Given that there is a scheduled auction and the coin has a starting price of 600 euros, it is likely that bidders will be willing to pay a bit more than the average hammer price, especially if demand is high.

Taking all these factors into account, an estimated maximum purchase price could be around 700-800 euros. This is based on the good condition of the coin and the potential interest from collectors, although it is important to remember that the final price will depend on the dynamics of the auction and the competition among bidders.