Hey Cheesefood,
I will try and answer your questions and keep this as short as possible. First off, great choice on honey (my favorite).
If you want a semi-sweet mel you should aim for four gallons (3 gallons water with your 1 gallon honey). This will give you a OG of about 1.11 . . . unfortunately, fruit adds little to the total % of fermentables (though it does add some good nutrients). For instance, if you used blueberries or cherries which are higher in flavor (compared to strawberries) you would want to use at least 2 lbs per gallon. Eight pounds of cherries would raise your OG to 1.117. This is about 15.8% potential abv.
The difference between sweet and dry mead yeast is their potential . . . sweet goes to like 12% and dry to 14% (ish). However, I have had horrible luck with both leading to stuck fermentations. I would recommend using either Lalvin D47 or 71B. Both are great for mel's (like D47 for cherries and 71b for blueberries, etc). They both are 14% potential so that would leave you with 1.8% sugar which is a semi-sweet (or off dry . . . a fine line). Either way, you can always back sweeten with more honey if it goes to dry.
Last bit of advice. I would split your fruit between primary and secondary. 1/3 in primary and 2/3 in secondary. In my experience this gives a better accounting of the fruit. If you go with five gallons and 12 pounds of honey it will ferment completely dry and you will have to sorbate/sulfite and then back sweeten to get a semi-sweet mead (but it will be ready sooner). Up to you.
Hope this helps. Good luck!