Hi Stephan
To some extent this is a personal choice driven by how you plan to use the boat- there are those that like lots of horses to cope with driving through significant waves and headwinds for hours on end, and others who use engines just to get on and off moorings. Is your 5 knots in flat water or in waves?
Through the years Atalantas have had pretty much every small engine you can think of. Popular choices have been the 9-10hp single Yanmar 1GM and the Beta twin 14hp. Both are higher than the Coventry Victor 12 or 16hp engines usually fitted when new, requiring raised floorboards. Note that petrol engine and modern diesel engine horsepowers are not directly comparable.
I have a 1GM in Helene. I have found it great for my purposes and find it happy to push the boat along at 4-5knots for hours on end even in a chop. Easy to service, smallish, However I have also been very impressed sailing (motoring) on Atalanta Mary with her 14hp Beta.
Another factor to consider in fitting an engine is access to the service points including seawater pump, filters, drain plugs, lift pump and bleed screws and so on.
Not sure if any of that helps….
Cheers
Nick