Much would depend on how/where/why the car has been layed up in the first place.
Many old barn finds were cars that were initially layed up as repairs were going to be more costly than the car was worth....... so why has the car been stored up and not used in the first place would be good to know..
Was it also during lay up either moved on its wheels or even engine started or any other maintenence been done while stored. For sure if not ever moved at all consider your tyres will be flat spotted, and battery knackered.
Then you have to consider has this car got sentimental value to you, and then what is going to be your long term life style. ie will the car just be used locally for going down to the shops or church on Sundays or will you maybe travelling longer distances or abroad.
Then the next question is how big is your 'pension' and how much of your kids inheritance do you really want to spend on the car both to bring up to safe usage and maintenance thereafter. Money well spent though IMHO rather than the tax man getting it.
For me, absolutely the very first thing would be body structure condition and I most certainly would be having a thorough inspection of the bodywork/chassis/cills/arches before I even considered spending a penny. Strip out carpets front and rear to make sure floor pans are good too. If you are expecting to have the car as an all year round car on UK roads it needs to be good metal.
However the key for any owner is also a question of what are your own personal skills and the items you are happy to do yourself and those areas where you are going to have to pay others.
It all depends really what you are starting with. Unless you have some idea or more facts to offer I would certainly not even attempt to start the car and beware of any local garage that will come along in HOPE and just jump start it.
I follow a long detailed particular procedure for testing out any old RR/B that has not been used or started up for years. Even after that though much can still be found faulty.
Let us know more what you are starting out with.
I looked over a beautiful Shadow1 last week for a friend that is now purchasing it. It was £15995 and we ended up paying £15k as cash buy. The car is near perfect and been well looked after. Had he not been a friend in all honesty I would have bought the car myself as it needs £zero to be spent on it.
Another guy has just spent over £25000 on a Shadow 1 body paint repairs, and now trying to sell it for £45k.......
it is on FS cars for sale...............no chance
Two cars of my 7 classic car collection will never see a profit in value during my lifetime from what they have cost me to restore, where as one car that cost me £645 nearly 50 years ago and has had probably no more over the years than £5000-£10,000 spent on it is today worth £90,000+
The RR/B S1 T1 have crept up a little in value but just not realising a sound investment yet to spend too much money on IMHO. I have looked after my SS1 now for over 40 years but more out of love as even doing all the work myself she has been a cash cow really rather than an angel.
All the best