6 houses bought and sold in Italy here! I’m resident and working in Italy.
I wouldn’t take on the buying or the building work without having a clear idea of duration of residence, Brexit tax situation, planning permission, building restrictions, tax breaks, and obligations.
At the very least you need an Italian geometra (surveyor) to back up your UK architect. Preferably local and ideally one who knows the town planning geometra. Ask your neighbours and at the comune. Find out if you are in a Tuscan “belle arti” area where building regulations are stricter. And factor in the tax you pay as non residents (seconda casa) both on the sale and rubbish collection, bills etc.
Your geometra will then find you reputable builders, plumbers etc who will work in the area. No use sourcing them yourselves! For renovations that improve the energy rating like double glazing, insulation and solar panels there is tax relief (if you pay Italian tax) or grants available. Guess who organises the compliance documents!
In the last 30 years of renovating and having work done, we’ve always bought top quality building materials, bathroom fittings etc, not DIY superstore, and the installation has normally been 1.5 times the materials cost. Maybe more nowadays. And you will almost certainly go over budget, even with quotes, because if a job’s worth doing…