I think that it’s completely ok to have requirements written in a document. Actually it’s great that you don't want to influence designer by presenting your own wireframes. In fact it’s his job to ask a lot of questions and come up with ideas.

But to make your cooperation smooth I would share every insight with designer. Everything you know will be beneficial for him. Access to the current product can be very helpful in understanding context, users' needs and most importantly - your business. Designer can also run an expert review on your product in order to verify weak and strong points of current solution.

Very important is to understand what UX Designer can do for you and not to fall into a trap of “popular misconceptions about UX Design”. So to better understand what is a role of UX Designer and how you can use his specific set of skills for your business, I recommend a great article - “How UX Designer can bring profit to you business”.

As for estimations and pricing, product estimations are always very specific. High standard designer/agency will ask a lot of questions, dig deeper into problem before full project estimation. It’s crucial to know what will be exactly needed, how much time project can take, what you expect etc.

I think that it’s completely ok to have requirements written in a document. Actually it’s great that you don't want to influence designer by presenting your own wireframes. In fact it’s his job to ask a lot of questions and come up with ideas.

But to make your cooperation smooth I would share every insight with designer. Everything you know will be beneficial for him. Access to the current product can be very helpful in understanding context, users' needs and most importantly - your business. Designer can also run an expert review on your product in order to verify weak and strong points of current solution.

Very important is to understand what UX Designer can do for you and not to fall into a trap of “popular misconceptions about UX Design”. So to better understand what is a role of UX Designer and how you can use his specific set of skills for your business, I recommend a great article - “How UX Designer can bring profit to you business”.

As for estimations and pricing, product estimations are always very specific. High standard designer/agency will ask a lot of questions, dig deeper into problem before full project estimation. It’s crucial to know what will be exactly needed, how much time project can take, what you expect etc.