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 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower conditions.

2. Auction History:
- The average hammer price for similar coins in the past is 147.24 euros.
- The coin has been auctioned 21 times, of which it has sold 17 times, indicating good demand.
- Previous hammer prices for coins with similar conditions (80-85) have varied, but some have reached up to 75 euros.

3. Starting Price: The current starting price is 200 euros, which is significantly higher than the average hammer price for similar coins. This may indicate that the seller has high expectations, but it could also limit buyer interest.

4. Price Trends: Looking at the hammer prices from previous auctions, it seems that coins in similar conditions have reached hammer prices ranging from 20 to 75 euros, with some exceptional cases.

Given all this, an estimated maximum purchase price could be set in a range that reflects both the condition of the coin and the price history. Considering that the condition is good and the average hammer price is 147.24 euros, a reasonable maximum purchase price could be:

Estimated maximum purchase price: 150-180 euros.

This range takes into account the good condition and the price history, but also considers that the starting price is higher than what has been seen in the past.