£5-20k is hilarious. There’s all sorts of costs you forget about: skip hire or waste disposal. VAT on labour. Decoration. Unless you’re a brilliant DIYer with time on your hands to learn and time to scour FB Marketplace and the like for secondhand bargains for fittings, it’s going to cost.
Great house with good bones, though – looking at it room by room to “modernise” one of those empty bedrooms you’d be looking at stripping the wallpaper, which may expose blown plaster, so replastering, rewiring and adding sockets, new radiator, refurbish windows, sand and finish floors or install carpet, paint and then curtains, furnishings on top. Bathrooms and kitchens obviously miles more expensive and complex. Plus you want to make sure the roof and walls are sound – always start from the outside in – boiler up to scratch, etc. Consider which room you could make quickly habitable to camp out in while working on/saving for the rest – that house is a years-long project if you only have a slim budget.
Bear in mind everything you do will uncover some other issue: we’ve just replaced all our windows and had our top-floor room stripped back and insulated and in doing so, discovered damp problems so now need scaffold and a roofer, plus some of the windows have revealed blown plaster behind paper, or other areas that need insulation, etc. Houses are like pulling a loose thread on a jumper: one tiny job leads to 8m bigger jobs. Just depends on what you’re up for and what level of hovel you can cope with living in for the end result. My hovel level is fairly high!
Also all the budget goes on boring “unseen” stuff like roof, wiring, plaster, waste, labour, etc, so all dreams of fancy tiles or F&B paint quickly get downgraded to B&Q and bog-standard brilliant white.