Do I need a gravel pad beneath skids on the finished shed

I looked at your profile, but don't know what part of the country you are in. Where you are makes a lot of difference. The building you are considering is a wooden structure, fairly heavy and can hold some cargo. Any building that size deserves a decent foundation. If you don't support it right, it will warp, shift and bend with the seasons, resulting in screwed doors and separations on the siding.since you said the ground can be wet in season, I would recommended a foundation that keeps it off the ground, a vapor barrier to protect he floor and is deep enough to withstand ground heaves if you are in an area where the ground freezes in winter. If you are in a non-freeze area, I think you may want to at least support the corners and side walls with blocks at least 12 inches in the ground. Any and all water you can stop from attacking your new building will increase it's life span. Your idea of a gravel base with supports would be a minimum. Let me know where you are, and I may be able to give you better advise.

I've said it before, and I'll say it again. It's tough to beat a 60 ampere sub-panel. You might be able to get by with a 40 amp setup, but it's always nice to have future expansion possibilities.

At 90 ft., you're probably okay to use 6 AWG copper, or 4 AWG aluminum conductors. You'll have to pull four conductors, either through conduit or via a cable.

Since it's a separate structure, you'll need a grounding electrode system. Which you'll have to bond the grounding conductor from the feeder, and the grounding conductors within the building to. You'll keep the neutral conductors isolated from the grounding conductors, which will likely mean removing a bonding jumper within the panel.