A 1.5 mm longsleve under the shortarm will be dramatically warmer! And boots will make life more comfortable as well. If you sail formula and don't swim much then hood and gloves really come into play in sub 10 C conditions.
For wetsuit durability you should focus on double lined suits, with a fabric also on the outside (surfing style suits). Single lined suits, and especially NP, are prone to get nicks. You could have a look at http://www.wetsuit.com
, they make particularly durable single lined suits (different neopren quality).