Hi Jeremy,
I do not believe the Fed will be hiking rates in 2019. The market is of the same opinion (https://www.bloomberg.com/news/articles/2018-11-21). Take that into consideration, as this impacts mortgage rates. Even so, you almost certainly will have a higher interest rate on a purchase at this time.
Keeping monthly payments down on the new home would primarily be a matter of using the right down payment. In your case, it sounds like if you used all the equity in you current home ($1.6-1.7 million - $775k = $825-925k), you could get the monthly payments close.
Regarding the custom home, if this is a tract home that you are customizing the finishes of, the sale price may be utilized for the tax base year established by the assessor (not appraiser). In this case, finishing the property with lower-quality finishes and renovating later could help with lowering the base year assessed value. If this is a stand-alone, custom-built home, then the assessor would likely utilize comps to determine the assessed value for the base year. In either scenario, the assessor will not take into consideration who paid directly for finishes, just whether or not they are in place at the time of assessment/sale, if they even have this information.
Brian Spohr
Double Crown Realty
DRE #02081921