There are positives and negatives to buying an ex-rental car. As you say, rental cars are usually serviced regularly and are more likely to longer journeys than frequent short trips. On the downside, there is no accounting for the kind of abuse it might have been subjected to, although this is arguably less likely for a more expensive rental such as a Mercedes.

It comes down to how well it compares in terms of pricing and specification compared to equivalent vehicles that are being sold privately or by a dealer. As long as the history is assured and it is priced competitively, it would be a sound choice.