ILR is permanent as long as you remain living in the UK, so you don't need to 'renew' the ILR, but if you don't apply for citizenship, you will need to get a new BRP card with updated photo/personal details every 10 years... same as you do with a photo driving licence.
Having said that, if you can afford to apply for UK citizenship, I would definitely recommend it, as it means you don't have to worry about ILR anymore, you will have the right to vote in the UK, and you will always have the right to live in the UK, even if you move abroad for a number of years.
The referees just need to fill out a form as part of the citizenship application.
I believe it asks for:
- how the referee knows the applicant (i.e. a short sentence stating how they know them) and their age and profession
- referee's name, date of birth and sex
- referee's address, phone number and email address
- referee's UK passport number
- referee's signature
You can see the paper version of the citizenship application here, which has the referee form on page 20-21:
https://www.gov.uk/government/publications/become-a-british-citizen-by-naturalisation-form-an