Hi.
im thinking about building a run-errands, grocery, utility bike from a mixte frame with a cargo fork like the one available from Crust cycles, with maybe a sturmey archer drum/dynamo front hub and maybe a 5-speed sturmey rear hub, also drum. The goal is to have a utility bike capable of doing medium-sized grocery runs and other dailies that doesn't have enviable ( attractive to thieves) parts when it's locked outside a store, but would also be basically bomb proof.

is this feasible? If so, what frame would you recommend, an old Nishiki or something else?

all suggestions appreciated.

A steel mixte is a great starting point. Grab whatever clean, rust free frame you can find locally rather than chasing a specific brand. The frame condition matters a lot more than the badge on the downtube for a project like this.