I'm also with the single engine diesel crowd and a newer boat. Just go up a few years and get a MS 390.... or the 340 or 400. All good boats, easy to single hand.
While the twin will cost you no more in fuel, and less than double in maintenance, some of the installs are pretty hard to get down there for maintenance and that's a major issue.
Some of the "musts"
1. Get a diesel.... no doubt about it!
2. For single handling, get one that's easy to get around, lower helm door.
3. NO ladders, get stairs.
4. With a single, get a thruster. I know the purests will say you don't need one, but buying it with the boat is SIGNIFICANTLY cheaper, and YES, you'll find it extremely handy.
5. Whatever boat you get, hire a captain that knows your boat to train you. Practice for awhile, and go back with him to brush up on what you forgot.
6. Have fun!
The difference in a 35 to 45 ft boat single handling is minimal. However, if your boating is absolutely minimal, rent a small boat (25 ft or so) and get some training on basic boat ops, wx, regs, limits, etc., etc. And take the online Boat US course (do it regardless).
And FWIW...... Just sold my twin engine gasser, no fresh water cooling, 2003 model. Great boat, but comparing it with my 40ft the maintenance, unexpected and regular was WAY more than the trawler. And the mileage WAY worse... and it was half as big. (however, fun)