I installed the current LAGAN back then for a guest kitchen (identical to RENGÖRA in housing and front door) into a FYNDIG kitchen.
With Knoxhult you don’t get a dishwasher front (just like back then with Fyndig). For white matte fronts you will definitely find a cut-to-size panel at the hardware store in the desired height. I would make this as tall as the neighboring carcasses above the plinth. You can no longer use the Knoxhult countertops; buy, for example, a corner piece countertop.
Two installation manuals come with the RENGÖRA, one for Metod and one for Faktum. You have to assemble according to the METOD manual (with hinge technology) and the drilling template may not fit perfectly; you have to adjust it by shifting it in height. I drilled several times for FYNDIG until it fit. You only need the front and a countertop; the rest is included with the dishwasher (sliding hinge bracket, screws, template, etc.).
Regarding the plinth, I would have a U-shaped piece made from three cut-to-size panels from the hardware store, cut slightly to the same plinth height as the original KNOXHULT cabinets, and stick these on the left and right to the neighboring carcasses with Tesa Powerstrips.