Just a few comments on fuel capacity - at a 25 knot cruise you can probably expect to burn at least 50 gph (0.50 nmpg) for a loaded boat in this size range. For your 100nm crossing, that would be 200 gallons each way. Add about 0.75 gph for a typical 10kW genset, and you are looking at another 54 gallons for 72 hours (3 days) use. You are now up to 454 gallons without allowing for any extra adventures or alternate plans dictated by weather.
I like to conservatively use a 20% factor for reserve fuel, because the last 10% can be junk that will play havoc with your fuel filters, especially as you are coming home and running low on fuel. Adding 20% reserve from 600 gallons gives you a total of 574 gallons, leaving only 26 gallons for "messing" around, etc. I would reccommend that you keep the reserve just for the unknown emergencies, especially if the Islands are fairly isolated..........
I would be more comfortable with a 700 gallon fuel capacity, and you will be hard pressed to find an older sportfisher that carries this kind of fuel in your size range. Looks like a Cabo 43/Bertram 45 would fit the bill, or maybe an IPS/Zeus powered Express that has better fuel efficiency.
Although I have not cruised on a Silverton 45, it does have a Donald Blount hull design and should be a good performer, even in the rough stuff. I would take a Cabo 40 over a Tiara 39 any day, and an old school sleeper would be a Topaz 44.
Just my $0.02