Sounds like you’ve put a lot of thought into this!
The Transit Connect seems like a great van. I do not have experience of owning one, but they seem better than most other car derived vans. It’s the on I would choose I think. Or a LWB VW Caddy, but they can be expensive as they’re more fashionable.
The problem with the pre-cut paneling kits is that they are normally designed for tradespeople and their vans. So the panels are cut to cover the holes in the interior panels, but they wont be cut to go to the very edge of the areas you want to panel. When I cut my panels I make them the exact shape I need, and then covering the carpet, etc is simple as the panel underneath goes up to the very edge of the interior.
It’s worth checking out the results of a vehicle with one of the kits, just to ensure you’re happy with it.
They can save a good deal of time though.
Carpeting the interior is the easiest option. Proper lining carpet is way easier to use, as it’s light and mouldable. Try these guys:
Other carpet works as well of course, but is heavy, harder to stick, and harder to work with.
I’d highly recommend using vinyl flooring or laminate flooring, as you can keep it clean really easily.
How a good quality sheet of 6mm plywood (9mm if you want to screw heavy stuff into it) for the ceiling? Get something with a oak veneer, and then wax or varnish it. It will be really light, fairly cheap and easy to work with. Birch veneer is cheaper and lighter colour.
I would go with a window rather than roof vent. Roof vents on their own don’t do a lot. A window is essential for a breeze in summer. Windows are less prone to leak also.
Your electrics sound perfect. Maybe go for 110ah battery if it fits, as it wont cost a lot more.
Get a good brand inverter, ie not the cheapest., but you probably don’t need to spend hundreds.
I’d recommend taking the bulkhead out. It just makes the whole space nicer, especially in daylight when you’ll want the light coming to the back.
However, I would recommend fitting a fairly thick and dark curtain instead. Make sure it goes edge to edge, and glue/stitch velcro to the edge of the curtain and stick it to the carpet on the sides.
The curtain will keep the light out when you sleep, it will look like a bulkhead from the outside (for security) and will keep the cold air in the cab, and warm air in the back (and vice verse in the summer, when you wake up to the sun blasting through the windscreen).
Sounds like a great vehicle in the making.
Have been tempted to get back into fishing myself this year.