Hello Chris,
I think you're on the right lines with "economy and simplicity". Too often one sees projects stalling or taking an inordinate amount of time and money because people over-reach themselves, try to do more than their skill level and finances permit, get carried away with gizmos and gadgets, aim for a work of art with finely fitted joinery where a more 'workboat' finish would take a quarter of the time.
Safety is an attitude of mind as much as a level of equipment. Keep the water out with strong hull, windows, hatches and ventilators; ensure reliable propulsion by both motor and sail; ensure reliable steering; get good new generation ground tackle (Rocna, Vulcan, Manson Supreme); know where you are (two independent sources of GPS position) and you're most of the way there.
Mast: the hybrid mast remains the pragmatic favourite. Find a 7in tube at a Cardiff aluminium stockholder and make a wooden topmast.
Sail: use one of Arne's stock designs, and go through his writings on entry-level rig building.
Interior fitout: exterior grade plywood from SE Asia, coated and bonded with epoxy. Paint it off-white or light cream, adding only the bare minimum of clear finished trim - it's this that takes the time.
KISS: Simple plumbing, with water drawn straight from 10 litre containers; basin to be emptied overside; composting heads; no other pipework. Simple electrics, a few LED lights; a sounder; a tablet and a phone for navigation, entertainment and communication; a VHF radio. The simplest of cookers. Don't bother with a heater for May to September cruising out of Milford Haven.