: Some specific questions, if'n I may: 1. I want a true sea kayak, but not very
: badly.

NOT VERY BADLY. Hm...I guess the symptoms you described would generally be considered serious by this board for sure, though not uncurable. D)
The first part of the cure would include wondering exactly what kind of kayaking program you would like to define: long expeditions or short trips, portaging or not, distant put-ins or padding in the vicinity, windy roads or not, big car and garage or small, etc.
The second (and last known) would be to start building anything at once!

: 2. What keeps 'em from heading straight to the bottom upon first contact with
: an oyster shell?

Truck tarp -not the leaky woven blue car stuff- makes good skins and is cheap worldwide. You can reinforce it with scrap stripes at the bottom -inside and outside the hull- and other points where the hull will rub when beaching.
It's a good thing to pack a roll of duct tape in case your skin would be teared.
It's used on folders. Polyester or ballistic nylon are more taut but suit non folders better.

: 3. Even though I have looked at folding boats and SOF yaks to some degree, I
: do not clearly understand how folders work, and following, the difference
: in complexity (read: expense) between them. Where might I learn more about
: folders without spending any money? Or at least, very little of it? Or
: should I ignore folders altogether, since compacting the boat isn't an
: issue at this point?

At this point, your choice; at a later stage, you might change your mind, depending exactly on what you want to do with your kayak.
About nobodys build folders (an early XXth Century German invention) nowadays, rather take-aparts.
Feathercraft from Vancouver showed the way -in 1981 IIRC- to use aluminium tubes fitting one into another thru smaller sliding connection tubes, which are riveted to one of the two connecting tubes. Now other manufacturers like Klepper or Nautiraid, do so. Tubes are forced into HDPE stations.
Tom Yost does a great job at giving this device all sorts of applications. Look up his site! You'll see ash-tree take-aparts by German builders, which allows Greelandic designs and swept-up bows as wood stringers are sometimes more flexible than alu, and ply gunwales let you build up sweeping, higher bows.
The advantage of a take-apart is easy storing; portaging, especially on long legs in an expedition as they can be back-packed; and transport as you can drop that bunch of light tubes tucked in a rolled pvc skin into the car's trunk, instead of cartopping that wind-sensitive boat before you run on the highway.
The skin is not as taut as on a non-folder skin-on-frame, as the boat has to be skinned then unskinned then skinned again, etc without slacking too much.
A T-A isn't generally much more expensive than a non-TA, though I guess not cheaper. You can do both material lists and price them.

: 4. Do I have to get a sewing machine, and learn how to use it?

If you build a take-apart, you just have to glue the seams along the chines, sheers, keelson and deckbeam with a 2comp glue.
Building a T-A is a tad more complicated as all fittings should be removable yet sturdy. Glueing two pieces of lumber together is basically easier, but you'll often have to peg them.
Permanent connections are also lighter, tho not to the extent it'll make a much noticeable difference from a traditional, one-piece replica.

Please click the Search function and look into the archives of this board: lots to glean!

Best, Eric