well, I d go a different route. Its not unusual to have the aspiration for longer cruises, but most people start off building experience locally, slowly venturing forth. During that time, they discover that their perfect boat isnt quite what they need after all, and change boat to better meet the new cruising dreams.. and so on, and so on.
So, I would look for boat that is economical and popular (so you can sell it on again reasonably easily), that would meet some of your cruising plans, but concentrate on a stepping stone of ambition. I m not sure anyone gets the first boat right, and you probably wont either ! So, I would go single engine diesel, coastal cruising for the first season til you know more what YOU want from a boat
Just one more minor point,you did mention that offshore is your preferred cruising location,just bear in mind that most bigger flybridge boats will prevent you from enjoying many upriver locations except perhaps the Thames due to airdraft,this means you will be missing out on some of the best locations in which to learn to go boating.
Good hunting.