Flaps - yes

Retracts - Absolutely, pneumatic only for me. Mechanicals might be very hard to install in a Spit where the gear are retracting outward. Unless somebody makes a mechanical specifically designed for this purpose of course. Either way I much prefer routing air lines to routing linkages but that's just my opinion.

Supplied hinges - They would most likely be fine but I prefer the CA hinges from Radio South myself.

Covering - Fiberglass and epoxy resin, fiberglass and polyacrylic, silk and dope would be the options I would consider. Last resort, iron on plastic scuffed and painted.