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

1. Current Condition: The coin is in a condition of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower conditions.

2. Starting Price and Previous Prices: The current starting price is 700 euros. In previous auctions, the average hammer price has been 373.68 euros, but there are also examples of higher hammer prices, such as 800 euros in 2018 for a coin with a similar condition.

3. Historical Trends: The coin has been auctioned 59 times, with an 89.8% sell-through rate (53 out of 59). This suggests there is consistent interest in this coin, which may influence its price.

4. Recent Hammer Prices: In the most recent auctions, hammer prices have varied, but there are examples of auctions reaching up to 750 euros for coins in a condition of 90.

5. Comparison with Starting and Hammer Prices: Given that the starting price is 700 euros, and considering that in the past hammer prices have reached up to 800 euros for coins in similar conditions, it is reasonable to assume that the maximum purchase price could be in a similar range.

### Estimated Maximum Purchase Price
Taking all these factors into account, an estimated maximum purchase price for this coin could be between 800 and 900 euros. This is based on the good condition of the coin, the demonstrated interest in previous auctions, and the hammer prices achieved under similar conditions.