Get a ski not a sea kayak. I used to work as a sea kayak instructor and have no bias against sea kayaks, but learning to roll can be an incredibly long process. To roll you have to have your face deep underwater and upside down with water trying to force its way up your nose, and then counterintuitively push your face even deeper towards the bottom of the sea to get it done. If you are very phobic about that whole feeling it may not even be possible to learn. remounting a ski is 100x easier to learn and you don't need an instructor standing next to you for hours in cold water. If you want to do multi-day expeditions with camping gear a sea kayak is the only way to go, but for everything else a ski is better. Also under stern rudders, even small ones, offer so much more control that overstern.
Re plastic vs composite, there is a significant performance difference. I personally think the performance benefit of a 23 vs 27 vs 32 pound composite ski is overstated, and the main benefit of that small weight reduction is ease of transport off the water. But the difference between a composite ski vs a 50-55 pound plastic ski is very real on the water, and the huge off the water. If you are just cruising around recreationally it might not make much difference, but if you enjoy the feeling of a boat accelerating from a paddle stroke, or want to accelerate onto waves to learn the addictive pleasure of downwind, a composite ski really is better. And off the water it is so much easier to carry and not drop a lighter ski especially when traversing uneven ground.
If your paddling is going to involve landing in many different random rocky slimy nasty places and banging the thing around a lot plastic might be the way to go. But most ski paddlers just figure out a few spots where they can safely get on and off the water. Have you considered a used fiberglass ski? Those are cheap and more durable than the ultralight composites, and would be much more pleasurable to paddle and carry than plastic. Keep in mind too that the plastic skis use a different construction than most plastic sea kayaks and recreational kayaks. Rather than solid plastic they are a sort of sandwhich construction with aerated plastic "foam" in the middle between solid plastic walls. They are stiffer than solid plastic, but the trade off is they are a bit more brittle and prone to damage from sharp impacts.
Current Skis: Nelo Vanquish AIR, Epic V10g4, NK 670 double, NK exrcize, Carbonology Feather, Think Jet, Knysna Sonic X
Former Skis: Epic v10g3, Kai Waa Vega, Epic V12 g2, Epic V12 g1, Epic v10 double, Nelo 550 g2, Fenn Elite S, Custom Kayaks Synergy