Lots to answer here!!!
I went down this road 2 years a go so hopefully can help; thought I'd buy a little boat as the beach fishing was pretty dire. Bought a 17.5ft Shetland (which I am now selling if you're interested!!) which was a great little boat. Could comfortably go out 10 miles on a still day - far enough to reach the good wrecks.
I would therefore advise firstly looking at what area you will be fishing and chose the boat accordingly. If you have great fishing within 2 miles then you don't need anything big, if you need to go out say 20 miles then obviously a bigger boat is a safer bet. Diesel is a good choice as it's a lot cheaper than petrol, about 30 - 40p a litre as you can use red diesel.
I think there is some false economy in buying a small boat, ie less than 16ft, (in our area anyway) as I've seen a few guys do it only to find that they can't go out that far (1-2 miles)and therefore can't reach the good fishing ground - then have to buy a bigger boat.
If it is your first boat I would recommend attending a boat handling course, I did one a few weeks a go as I have just bought a bigger boat - well worth the money and wish I had done it sooner, costs about £100 a day with a qualified RYA instructor on your own boat.
Morring fees some to vary but where I am near Brighton I pay about £1000 per year for a 23ft boat, Brighton Marina would have been nearer £1600, but more facilities.
You should have insurance, but this is quite cheap, mine was £160 a year. There is no need for a license or registration unless you go over to France at which point the boat should be registered with the Small Ships Register (SSR).
Remember the importance of the VHF radio, GPS etc - vital equipment on any boat - but all add to the cost!!
Hope this answers some of your questions Ant.