Personally I’m warey of an engine that has stood a number of years and would automatically strip it down. On other bikes engine seals have regularly been know to fail on bikes that have done very little mileage and they can become brittle. The mixture of damp and a dry sump where roller bearings have rested for so long would make me warey.
I’m not trying to put you off the purchase but if you’re looking to just get on and ride with the minimum of fuss, a bike that has been run regularly with more miles can sometimes be a better option. If the bike has rested on the tyres for a long time they can leave a flat on the bottom that will be noticed at any speed. I believe it is very good advice to change the tyres. The modern rubber will be much more supple and offer better grip. The old rubber will have hardened.
If nothing else these points will provide good bargaining power when negotiating a deal.
There is a new “old bike magazine” called RealClassic from the long standing editor of the Classic Bike Guide before it was bought out. Issue 3 has a write up on a Silk by the Toaster. If you try out the site http://www.realclassic.co.uk you can go on the message board and leave a posting there as well as here. Toaster (Steve) is very helpful and his mate actually owns one and I’m sure he will give you some good advice. Just mention my name.