Just skip the trusses altogether and go with rafters and and an attic.
If he wants clear span, I doubt that is an option.
There are several options for storage trusses.
Spread-Web Trusses are often designed for storage and give some, but not a lot of storage space. These probably allow the lowest roof pitch and are probably the lease expensive.
Room In Attic (gable) Trusses will give more space. Depending on the desired room size (height and width), the pitch may need to be pretty steep. These trusses are often designed for load as a living space.
Room In Attic (gambrel) Trusses are like room in attic (gable) trusses, but have a gambrel (barn style) roofline. These will probably provide the largest space, but will have a very steep lower pitch (generally at least 12:12) and may or may not be a style that fits with your property.
All these options can be easily designed for 30' span and various loading.
What I realized as i was planning my build was that the money I spent on trusses gave me some very inexpensive space (maybe $2-3/sqft extra for trusses and subfloor). Also, the price of the trusses seemed much smaller as I started adding everything else up (roofing, siding, concrete are my biggest items).