Why don't you take a look at the ZAR range of boats. You might find they give you a good balance of what you are looking for - the sea keeping of a RIB with the comfort and practicality of a sportsboat. The website is http://www.zar-formenti.com
I think your £17k would buy you a new 4.7 with 50 HP Engine and Trailer:
Worth bearing in mind that ZAR measure their boats differently form most builders, so there 4.7 is equivalant to a 5.something from most other RIB builders.
To clarify mu position, we do some training, handover and deliveries for the UK importer. We have found that the uniquely shaped hull is ideal for our choppier Solent conditions, giving a very composed and dry ride. Very stable at rest too so ideal if you want to have lunch aboard etc. We have talen these boats cross channel no problme and so far the owners I have met have all been very happy
The guy to talk to is Dan Cross, he sometimes posts on here and is very helpful.