Claudio,

I've only bought in the Uk once but have driven over and back a few times.
If you're buying in England (excluding Devon / Cornwall) Holyhead is the best port to use as you get straight back to Dublin. If it's Scotland the do Stranraer to Belfast and come down the m1.

I brought cash when I bought my car but I'm not sure if you'd want to do that for such a large amount. Others who have done in can advise on that, I'm sure.