Did similiar a few years back in my transition to IP cameras for home use. Purchased Grandstream IP camera boards (liked the SIP functions). Retrofited these boards in outdoor dome cameras removing the old legacy boards and heaters in the domes. Used them outside for a few years and they worked fine.
Overdid it on the mailbox. Neighbor suggested I DIY it after getting some quotes. ($3-$4 k). Built a chase to the mailbox and from the mailbox also ran 12VDC for LED low voltage lighting. I also installed cams facing the house in the back yard berms.
Doing similiar today (for tinkering). Found a source for Hikvision 5MP boards for $25 (or less). Retro fitting these boards in old new stock Speco and Everfocus domes.