First of all, there is no in person interview. In fact, much of the time there isn't an interview at all. If they have questions about your application or it is randomly selected, then they arrange a time for a phone interview. So that is one expense you won't need.
In terms of proof, they list what you need on the visa site. But I think that it would be advisable to get your arrangement in terms of living conditions in writing. You will also need to consider if there is sufficient room for one more person. To prove that you can get a letter (I think it is from the council, but someone will chime in if I'm wrong) stating how many people those accommodations can handle. Another way and the one we used when I moved over and we stayed with my in-laws is to draw a lay-out of the flat/house with approximate measurements. It does not need to be perfect though.
In terms of the benefits thing you can receive them but you need to show that he won't need them. You can do that by showing that he or you have savings until he can find a job...and showing that he is employable by including a CV and/or listings for jobs which he is qualified.
Good Luck.
