For that kind of house etc i would go for a homebuyers survey, while not a full building survey it is quiite detailed. A valuation isnt worth the paper its written on, its also only for the lender and no real use to you
Theres not point going for anything less, you dont want to buy the house and discover something which costs £££. You might as well spend about £200-300 now and save yourself any possible heartache
With regards to the roof repairs you are some what mistaken on insurance. Wheather or not you have roof problems they wouldnt be covered as the insurance would only cover it if for example there had been high winds to dislodge tiles(the resultant water ingress damage may/may not be covered). People often make this mistake thinking insurance will cover anything and then get a shock when thier claim is repudiated.
You should also read up on house insurance etc