Your goal to be at the top of the SERP is unrealistic. Search results are not static and depend on many variables, e.g., the user’s location, the size of the info input display, the queries of the current session, and so on. For a business, a more realistic goal is to increase the traffic of target users, which are mostly converted into customers, that is, to increase sales from the online visibility of the business and the products you offer. The search phrase Sell my house is too generic and, if fully optimized, can bring tons of traffic to your website without wasted traffic with zero conversions.
The price and the time depends on many details that create the required amount of work and can range between 500 and 10,000 euros.