Re: your question- ARF's aren't hard to put together. For bigger planes, the cost of shipping a fully assembled model is pretty high. So the ARF route makes sense. They aren't hard to put together. All you have to do on most is glue the tail on, install the landing gear, and install the electronics and power system.
I think you are going the right route with gas. Electric will never match wet power for flight time, in part because your power source is burning off as you go.

For those who are suspicious of the OP- the concerns have been expressed and the OP has responded. His responses are reasonable. It's not like we are giving out some super secret information here. Plenty of people have done long flight time airplanes before with no ill effects. I do understand the concern about autonomous flight and the ways that it can be used for nefarious purposes. But let's not assume bad things automatically. Of course, anyone who isn't comfortable helping the OP can choose not to post in his thread.