The boat itself, foot passenger cost is only about £20, and the boats to Scotland with a car are usually about £110. £130 in total for boats.
It's not that scary driving it yourself, I've done it loads of times
It is about 60 miles from Belfast to Bushmills, about £6 - £10 in diesel depending on how you drive it.
Just remember to get her insured. I once was halfway up the motorway before I remembered I hadn't called them
Only other thing I can see, does it come with an MOT? It is 2006 so due its first NI MOT this year. If it has one it will be displayed as a disc on the windscreen. If not, as it is >3 years it will need an MOT before driving in GB.
NI MOTs should be legal, I have taxed cars in Scotland before (though the post office did end up looking it up and down).
As far as the V5 goes, technically it is an import, so you will need to fill in a different part of the form. Make sure the seller knows this too.
If the DVLA are as useless as the DVLNI, then they will post you a form asking for all sorts of figures like the dB noise of the exhaust

it is easier to go to the local tax office and they should just ask you to fill in the basics such as VIN etc. as the vehicle is already EU / UK import approved.
You will need to send your NI tax to Swansea, they should swap it for a GB disc with the same date.
Importing from GB to NI I was given the chance to change the plates, I would imagine NI to GB would be similar. Keeping NI plates would hide the age / look a bit "chavvy" / look a bit touristy depending on your own views. I once had someone ask if my name was "MAZ"...