You've come to the right place! I can talk for hours about what you asked

Estimating is tricky and is the key to weather your business will survive or fail. I won't give you very specific advice but I'll break down the estimating process for you.
Material+Labor costs+permits=job cost. Job costs+overhead+profit= what the customer pays.
Rule one is know your overhead. Ya know? Those pesky bills that come every month no matter if you do 0-100 jobs. Telephone, insurance, car payments, office rent, secretary etc... What we do is break our overhead into working days. We know we work about 200-225 days per year. All jobs are then broken down into working days and the overhead value is multiplied by the number of days and added to the job cost. At that point we have figured the very least you can make on a job and BREAK EVEN. Any money after this point is profit.
Even if you guys are working as installers don't forget to figure your time into thr equation when estimating. Figure what youa re worth per hour. Figure how many hours the job will take. Don't forget to ad labor burdens like insurance and taxes to the number. This is how you figure labor cost.
As far as the partnership goes: I don't know your relationship with your future partner so I can't comment but I will say that you should enter into the partnership with a formal agreement drafted by a lawyer. The lawyer will identify who has what responsibility and what happens if someone wants out. This is a must, no matter how much you trust the person... People change.
One more tip on estimating... It's better to lose a job because you are too high than win a job because you are too low. Last tip, use a spreadsheet in exell for figuring the math. It might take some time to construct and tweak the spreadsheet but it's time well spent.