its exempt from R2FR as he is resident landlord.
its 10k for the freehold how much per flat depends on how many participate and as posted that on sale he would grant a similar lease and rent to the incoming purchaser. The average buyer would be hard pressed to find a lender as they will not lend on a freehold with a retained unit - a flat without a lease. In most cases he would be best off selling the freehold to one party and the flat to an owner.
In most cases the residents are the one who would pay the most as they have the most to gain or as some would say the most to lose, if they are saddled with a commercially minded landlord who appoints and agent incurring their fees as well as the service charge.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers.