Actually, I made the initial inquiry to a few companies.
So let me make it clear. It looks obvious that you need an expert input. Not all experts are the same: some are very good, but some are not... And it comes with a fee. There are 4 types of fees:
1) % of your budget. Say it can be 10% (in reality, from 8% to 15%). So if you plan your advertising budget £2000 per month, you pay the fee of £200. Obviously, a larger agency will expect budget from 10000 at least (and fee from £1000).
2) As a flat monthly service fee, not connected to budget. Say, £350 per month. Usually you will be asked more in the first month or so, and less after that (maintenance and further tuning)
3) Per result agreement: suppose it is worth for you to pay up-to £18 per 1 phone enquiry. Then an agreement can be: "£10 per each 10 enquiries of up-to £17", plus a bonus if the expert can get the price lower.
4) Simply pay per each enquiry, say £15 per enquiry. "Pay per lead" (In the USA, there is a huge market for this).
Larger agencies usually work on type 1 (% of budget).
Many smaller ones will agree on type 2 (monthly flat fee).
You will have difficulty to find type 3 (fee per result), it is unusual in the UK. (I get more clients in USA. To me it looks as the best for the expert and business, but I am obviously biased
Type 4 is available only for certain kind of businesses.
So if you hint what keyword will be typical for your business (say, fancy dresses, plumber service, or sport shoes etc), it will be easier to advise further. For type 3, you can suggest what do you think is the reasonable top expense per 1 enquiry (and/or per 1 actual order).
The experts here will provide you with further advice, much more effectively, after that…