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

1. Starting price: 225 euros.
2. Average auction price in the past: 420 euros.
3. Condition of the coin: 70 (MBC-), which indicates good preservation, although it is not the best.
4. Auction history: It has been auctioned once before, and it sold for 420 euros.

Given that the average auction price in the past has been 420 euros and the coin has a condition of 70, which is quite good, we can infer that interest in this coin could be high.

Considering that the starting price is 225 euros and the average auction price has been 420 euros, an estimated maximum purchase price could be set within a range between the starting price and the average auction price.

A reasonable approach would be to establish a maximum purchase price that does not exceed the average auction price but also takes into account the starting price. Since the starting price is 225 euros, a maximum purchase price could be slightly higher than this, but not as high as the average auction price.

Therefore, an estimated maximum purchase price could be around 350 euros. This allows for a profit margin for the seller and remains within a reasonable range based on the auction history.