Based on the provided data, we can conduct an analysis to estimate a maximum purchase price for the coin in question.

1. Starting price: 6000 euros.
2. Average hammer price: 8000 euros.
3. Condition of the coin: 90 (EF), indicating good preservation.
4. Auction history: It has been auctioned once in the past, with an estimated price of 5000 euros and a hammer price of 8000 euros.

Given that the coin is in good condition (90) and has reached a hammer price of 8000 euros in a previous auction, it is reasonable to consider that the hammer price could be in a similar range or even higher, depending on the demand in the current auction.

Since the starting price is 6000 euros and the average hammer price has been 8000 euros, an estimated maximum purchase price could be around 8000 euros, as this is the price that has been reached previously and reflects market interest.

Therefore, an estimated maximum purchase price for this coin could be 8000 euros.