The only way you could get normal Insurance is via the vendors existing Insurance company. You would need their policy number, name of Insurers etc, so that you could contact them. If the vendors have registered a claim against the current policy, you should also gain the Insurers and vendors acceptance for you to take over all rights for the claim registered under policy no. 123456. The Insurers may have a form for this, to send on to the vendors. The negative is that you will be stuck with the same Insurers year after year, as the premium goes up. Read of one case recently where someone started off paying £300 and 3 years later the Insurers increased the premium to £1200, which they could not afford.
The alternative is for you to obtain Insurance via a specialist e.g Bureau Insurance services or Home Protect. But under this arrangement, no pre-existing damage would be covered and in the event of a claim, you might have an argument on your hands. The argument being whether any damage happened before or after any policy was arranged.
For this type of property, make sure you negotiate a substantial discount from the normal market value of a similar property that has not had any subsidence damage. Put some money to one side, so that you have funds available should the need arise.
In regard to obtaining a mortgage, that might not be that straightforward either. They might not lend you more than x percentage of the properties value, to minimise their risk.