I would add a contingency line, but not use it as a substitute for answers. Mark every amount as confirmed, estimated or still unknown, and record who is confirming it and when it becomes payable.
Most importantly, settle purchase eligibility and ownership restrictions before spending heavily...
The lender may prefer a comparison across the full 15 years, but I would hesitate to use that as the only horizon. Base the main calculation on how long you might actually retain this particular mortgage, then run an earlier-sale scenario as well.
For each period, add interest, arrangement...