Gosh so many responses and they all pretty much reflect my dilemma's really.
So @SilentHedges @UsingChangeofName @Floralnomad @SilentHedges and others who've taken the time to respond with your ideas (thank you)
So - to clarify. I guestimated the sum of £100,000 to modernise partly from experience of buying and having work on houses but also based on the soaring costs of materials and labour. DS works in the building trade and we've talked about this at some length, he didn't think £100,000 either but OTOH said they are all constantly being told to be very careful to get their measurements/cuts etc right first time so as not to waste materials which are rising in costs very rapidly.
As examples, when DF was alive last year I had some reparative work to a bit of the plaster in the small bedroom (which can be seen in the picture), a curtain rail replaced, a deep clean and a couple of other bits done - cost, £2,200. I had part of the purlin in DF's roof replaced as it was split and that, together with repointing some of the ridge tiles, cost over £1,000. I also have the mindset that modernising would be to a quality/style which I'd want so I'd feel like it's my home. However, these are the things I know need doing:
The felt roof on the upstairs windows is long overdue replacing (hasn't been replaced since the extension was put on) so scaffolding plus materials plus labour to keep it waterproof and insulated. I have no idea what that would be so probably plucked a sum out of my head.
The windows are all really old and some of them don't open anymore, there is only one key for all of them anyway. Ditto the front porch door and inner front door. In my tiny house new windows and doors cost me about £7,500 just before the pandemic.
The bathrooms are actually ok, just need updating and both showers are really very good quality - but to update (according to things I've seen on MN) pretty expensive, I reckoned in the region of £15 - 20,000 as they're both very big rooms by today's standards.
Kitchen, everything works it has gas and electric but to put a new kitchen in, god knows how much, with all appliances I'd have thought about £15,000 +. However, not essential.
Ceilings skimming over the artex which is on every ceiling. Not sure how much plasterers charge for that kind of work, I thought maybe around £1,500
Some of the electrics don't make sense and sockets etc need adding or moving plus other more cosmetic things. The last time I had an electrician out it was very expensive!
The outside of the house has some asbestos tiles which will either need removing (they're ugly even if it's the less harmful asbestos) or cladding over.
THEN:
The garage needs some repair and making good to the roof (also asbestos) and the car port is rickety. I wouldn't use the garage.
All the concrete drive is uneven, patchy and really ugly. I hadn't factored in the costs of making it good or replacing it but it's awful. It doesn't NEED replacing though but to do so would be very expensive I'd imagine.
Then there's the cosmetic stuff of decor and other fixtures, flooring, new internal doors, all the bedrooms have built in wardrobes along entire walls which is great (I mean acres of storage space) but they are probably from the days of MFI.
I worry about renting the house out as you hear so many awful stories about bad tenants.
Then there's the reality that I'm grieving and I actually just retired from the NHS because what with how things are I was beyond burned out and I had nothing else to give. I can carry on working privately as I'm a young 63 but I'm literally just in the first couple of weeks of leaving.