Purchase tennants in situ

Other

Hello we have made an offer on a property. This will be my 2nd buy to let property.
The property has tenants in situ. A married and working couple who have been in the property 3 years.
We understand the tenants have been served notice but we have not seen this.
We have spoken directly with the tenants and they would like to stay.
We would like to sign them up to a new 6 month agreement (at a slightly increased price) - they are comfortable with this.
After the initial 6 months we would then look to increase the rent by a further %C2£35 per month. We have advised the tenants on this and they are comfortable.
The property has recently had electrics, gas checks and has certificates and an EPC Cert.
There is also a full inventory list from when the tenants initially rented the property - which we will go through in detail with them and update.
We would like to use the NRLA resources to set up the new tenancy.
Please could you advise on steps we need to include in respect on tenants in situ, any considerations we need the solicitor to be involved with / check and any recommendations.
Also please advise if existing gas and electricity certificates will still be valid or if l will need to get these redone - the electricity one was only issued this week.
Any wider recommendations re tenants in situ would be recommended. We have meet them and understand why the current landlord is selling and feel that they are good tenants.
We understand their intention is to be in the property got another 18 months.