I had a 2003 Nacra F18 that replaced a N6.0 NA. I thought the difference was dramatic and really enjoyed the improved ride of the F18, big smiles!!!! It should be even more so relative to the 5.8.
Many of the newer F18s (and F16s) moved the front beam back 1-2 inches, flattened the bottoms, and move more volume lower in the hull; resulting in the ability to drive downhill harder. Though the changes seem slight, the difference is significant when you are pushing the boat to the limit.
For club racing, you'll be fine on the 2005 Nacra F18. You'll get a few years out of it, improve your skills, and be in a good place to get the latest/greatest with the next iteration of improvements and leap frog everyone else that has today's best