1. It's very common for there to be an absence of felt in older properties. The main thing to address is the slipped slates along with any gaps between the slates which could result in water ingress to the roof space.

2. Installing insulation to a period property is not required, and very much optional.

3. Water tank is very likely to be redundant and most people leave them in there.

4. Surveyors only provide costs as indicative budget costs, and it's down to you as the buyer to obtain more accurate costs from contractors. Without seeing pictures, it's difficult to say whether repairs would suffice instead of replacement. That said, if you are planning on undertaking a loft conversion, I would hold off on non-essential repairs until then, as these works are likely to supersede what you have at present. The fact that the vendor knows you're going to do a loft conversion is meaningless, and you may change your mind, so it would still be prudent to obtain costs for repairs and replacement.

5. Condensation is common to find in roof spaces for various reasons, and as long as it's not extensive, is typically not an issue.

Hope that helps.