I think up to £25k is realistic for that work. The good news is that if if it is in a state you don't need to worry about making more mess by renovating!
Our house was disgusting when we moved in. I wondered if they had been chopping up bodies given there were blood stains on the carpet..... Seriously, it was horrid. A good clean when we moved in instantly improved it and from there it was nice to bring it back to life and choose everything for myself.
Plasterers have really dropped their rates as work is thin on the ground. I had conservatory taken from bare brick to fully done for £200 and my friend is being charged £40 per wall for skimming.
You could easily do skirting and architrave yourself if you invest in an electric, circular saw with an angle guide at the base. Ours cost about £40 and even I can cut skirting with it and I'm rubbish at anything requiring accuracy.
Decorating yourself will save lots of money.
My sister has just paid £2,000 for new fencing on her detached property and it has made a real difference.
The only thing I'd be wary of is the garden..... Ours was a mess and we intended to tidy it up ourselves but in the end we had to pay a landscaper as it wasn't even ground and was so overgrown it was just too much for us.
Taking a builder is a really good idea. As cuddy says you could get a quote for the lot and look to reduce it by doing some work yourself if it seems too expensive.
You could get a full survey too which would highlight any hidden issues?