1, Its fairly straightforward to convert from G to 2 Reg but I would let ASG at Guersey deal with it as theyre used to doing it. Also the aircraft has to be inspected by 2 Reg surveyors and its easier, quicker and cheaper if the aircraft is in Guernsey. You will have to create an acceptable maintenance programme. This can either broadly follow FAA rules or EASA with 2-Reg nuances (again ASG have done and pleanty of these and acceptable to 2-Reg. I strongly recommend you follow FAA rules and they are generally not gold plated like EASA and therefore cheaper.
2 If you want the aircraft to be in free circulation in Europe and you are not a resident of Guernsey you will have to pay VAT.
3. Disadvantages - You can elect to fly on FAA or EASA licence but your licence has to be validated by the Bailiwick of Guernsey. Costs about £80 and lasts 2 years. Every maintenance organisation/engineer that works on your plane has to be 2-Reg endorsed, this costs about £120.