Aidan,
Converting a 5.5 uni to a sloop isn't as difficult as it may seem. basically the difference between the uni and the sloop are a different mast, stays, main, and obviously the jib. Nacra used to offer purchasing a sloop rig which I believe had a shorter mast than the uni, the front only has one stay versus the two that were on the uni, and the main was smaller.
I raced a 5.5 sloop from about the time I was 9 till I was 14. Its a great boat for just screwing around on with the family or going out and racing. Its a little bit more stable than the 5.0 as well.
Leah