I'm afraid I'm going to be a bit useless with the whole exotic climate thing, but I can offer an opinion on the engine.
There are two trains of thought wrt horsepower. The decision basically boils down to "enough to push you along" vs "you never know when you might need the extra".
Having run with an Avon SR4 / 25Hp (max rated 50) I only once wished I had a tad more grunt, but all I did was drive appropriately to my craft & conditions. The other way of looking at it is that the bolt holes to fix the 90 & 115 to your transom wil lbe the same regardless of engine make (one fgood thing about standardisation!), so if you go for a 90 then decide more gee gees would be better, it's a relatively simple job to upgrade.
I've not had a chance to look at the two proposed power plants, but sometimes the extra few horses are nicely offset by a crew's worth of extra weight, which starts to become significant at the "five & a bit m" size of boat. (to put it in perspective I uasd to have a Yam on my transom, weighed in at about 110-ish KG. The Merc clocks in at 83Kg, and it now planes at 15 knots without needing the extra "throttle blip" to get it over it's own bow wave. 18 was about the minimum with the Yam.....
I'll now hand you over to the Medline owners for talk about seating layout & sunbathing!