To estimate a maximum purchase price for the coin that is going to be auctioned, we can consider several factors based on the provided data:

1. Starting price: 70 euros.
2. Coin condition: 70 (MBC-), which is a medium condition.
3. Auction history: The coin has been auctioned 54 times, of which it has sold 40 times. This indicates good demand.
4. Average starting price: 371.36 euros.
5. Average hammer price: 502.97 euros.
6. Previous hammer prices: There are several past hammer prices that varied significantly, but some of the highest are 2400 euros and 2651.4 euros in better condition.

Given that the condition of the coin in question is 70 (MBC-), which is lower than the higher conditions that have reached very high hammer prices, but still an acceptable condition, we can make an estimation.

Considering the average hammer price of 502.97 euros and the fact that condition is an important factor, a reasonable maximum purchase price could be in the range of 300 to 500 euros.

However, since the starting price is 70 euros, and considering the demand and sales history, an estimated maximum purchase price could be:

Estimated maximum purchase price: 400 euros.

This price reflects a balance between the condition, price history, and current demand.