i'm having a lot of fun soloing my nacra 5.2 on a small inland lake
at 75kg i'm a bit too light to right it without aids but as i'm not in the open ocean a flip should never be a life threatening situation
flipping a 5.8 at sea solo would be a bit of a gamble in my book......

for soloing at sea 17foot, 5.2 meters should probably be your limit
in that price range boat condition is likely to be more important than boat make or type

continue to do as much research as you can look but basically buy the best overall condition boat that seems to meet your needs
otherwise you will only get a little sailing on it before needing to spend large sums on things like sails, blocks, hull repairs etc.