Update Letting Agents Joomla Website - Possibly New Site Considered

I'm working with a client who is a letting agency in the UK. They have had issues with their current digital agency and are seeking to move away.

They would like to keep the existing website but upgrade it from Joomla 1.15 to a newer version and also fix some issues the current site has.

We're looking for someone who will be able to advise and implement all the changes. Ideally would like to work with someone UK based but cost is most important factor here.

Please respond with a ballpark of cost, previous work and any information we may find useful.

Feel free to ask any questions.