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

1. Starting price: 70 euros.
2. Average hammer price: 98.9 euros.
3. History of hammer prices: There are several previous hammer prices that may be relevant. For example, in recent auctions, hammer prices have varied significantly, ranging from 20 euros to 400 euros, depending on the condition of the coin.
4. Condition: No information is provided about the condition of the coin in question, which makes the estimation more complicated. However, the average hammer price suggests that the coin could be in acceptable condition.

Given that the average hammer price is 98.9 euros and considering that the starting price is 70 euros, a reasonable maximum purchase price could be slightly higher than the average hammer price, but not too much, since the starting price is relatively low.

Estimated maximum purchase price: 120 euros.

This price allows for a margin for competition in the auction, considering that other bidders may be interested in the coin.