I would say:
...to an acceptable standard...
...to a high standard...
Quality is achieved though good workmanship and use of the right materials. The purpose of a contract such as this is to spell out exactly what is meant by 'proper quality', 'high standard' etc which are rather vague expressions. There is often an industry-standard specification for materials in order to achieve the highest standard of work.
For example, if a builder were constructing a fence, you might specify '...using pressure-treated water-repellant timber posts size 100 x 100mm, with panels fixed using galvanised nails and aluminium security screws...'
Hope this is useful.
Student or Learner