Well as someone who is going through a septic project currently (at my personal residence) I will chime in with what I know so far.
First you will likely want to contact your county sanitary department (search "county" POWTS) they should be able to guide you to a county licensed inspector. I see you mention the pumper inspected it (if they also do installs I would try and find a different inspector to get an opinion of someone that doesn't stand to make money on the new system).
You will then need to locate septic designer. I called a local excavating company that does septic installs to get recommendations. The septic designer should be able to guide you in planning the new system based on your local regulations. (In my case the septic field is failing and my property is too close to the water table for a conventional septic to meet code. I have to install a mound system. This is also the step I am currently at due to travelling for work. I need to find time to meet with the planner.)
Finally you will have to have the plan to get an accurate measurement of the required materials so you can get a quote from the installer (usually and excavating company).
The excavating company I plan to use said they can usually quote the same day they they planner is onsite.
Supposedly the previous owner received two quotes for a new mound system($10,200 and $10,500) however, I'm not sure that a formal septic plan was made and these may just have been ballpark numbers. I probably should have pressed the issue harder when I purchased but the issue was discovered one week before close and I could not extend.
I will update this thread once I have more information (hopefully next week I can schedule the planner)