Believe the dealer is being way more than a tad optimistic with "5hp would be ample".
I have a Y9.9 4st elect start on a Columbia22 just bought and it is a big hunk of machinery see below - am going to change it out for a smaller one. A Y9.9 may not fit in your well. But the quietness of the thing is amazing - at idle I thought it had stalled - first 4st oboard I have had. Maybe I am a bit deaf or worse. Do you want to buy it?
The T9.8 looks good at 37kg - weight depending on shaft length.
Check the geometry of the well etc and the shaft length needed, also do you want elec start or just rope pull manual start - believe the Y9.9 is the smallest in their range with elec start. Most oboards 6hp and more have chargers but if you have a solar panel this is not needed.
Also be aware that raising and lowering too heavy oboards in yacht wells or on transoms can be a real pain, can do your back in. Tackle systems can be rigged up to lead to winches but securing systems - which hold the oboard down or up - need to be activated also.