* Size max 6.5m for reasons of towing and launching, Sensible
* min 5.75m for reasons of ride and general robustness. Also endurance (as in miles before filling up - sorry, I suppose that should read nautical miles) - bigger boat more fuel - or am I wrong there Sensible. Go for the biggest fuel tank you can (ours is 270 litres) and preferably underdeck (IMHO)
* seating 4 jockey seats - plus a small bench in front of console (terminology?) for occasional fair weather use Sensible
* engine size - don't know, assumed the builder would tell me Boats are rated for a maximum horsepower and/or weight. For a 6.5m I would say you need a 115 - 150hp engine
*2 stroke or 4 stroke - don't even understand the question...Generally speaking - 2 stroke = noisier & more expensive to run but more acceleration/speed; 4 stroke = quieter & cheaper to run but not as fast. Of course you could opt for diesel - more expensive to purchase but cheaper to run and higher resale value.
(running for cover, anticipating the bullets...)