So I have decided that I need to build a new ramp. My question is, "Is this a project for a beginner or am I out of my mind?"

Go for it - what's the worst that can happen? You end up with a weak
or broken ramp? Then you're in the same position you're in now. What
I would do: try to err on the side of overbuilding it - 2 layers of
3/4" plywood, a few 2x6 "studs" underneath (everything pressure
treated/exterior grade), figure out what angle you need to cut them off
so they're flush with the shed. If I thought it necessary, I'd sink
some heavy angle-iron or pressure treated stakes into the ground at the
bottom end of the ramp, fasten securely to bottom of ramp, cut off
excess stake. Use all stainless fasteners.
My father had a thrown-together ramp leading into his shed for years
that wasn't nearly this carefully constructed, so something like this
should last quite a while.
I'm sure this wouldn't meet any kind of code, but it's not a dwelling,
and I doubt there are any codes for ramps leading into storage sheds,
and even if there are, I doubt if anyone would care. I'm sure someone
will correct me if I'm wrong on that.
Good luck,
Andy