Starcraft and Sea Nymph are comparable quality but the Sea Nymph hull shape and high ransom are a plus (harder to find used long shaft small outboards here).
The 6 hp is not adequate for those boats. There is just too much water to displace on those hulls.
Have had 6, 7.5, 9.8, 18, 20, 25, and 30 hp outboards on my Starcraft 14' (which is similar, or the same as the one pictured). The 9.8 is ok with one person but it was a nearly new one (max speed is less than 15 mph). A 9.5 will be tedious with more than one person or a load in the boat. Understand you're not concerned about top end, but you will only get on plane with a minimum load and less than two adults. Post 1984 outboards for Johnson would be a different story as the hp is rated at the prop, as opposed to the crank
IMHO, pre-1984 15-25 hp is ideal for those boats. I like the 25, as it works good in all ranges and will get the boat going 25-30 MPH when needed.
That powerhead down low into the lower cowl design of Johnson 9.5 is a pain to work on.
An good trailer won't distract from the pleasure of an outing, others will.
Good Luck