Greetings Ahmed,
For DpnI, you need to use a strain without dam or dcm methylases, NEB has one strain (C2925), but that is more like a cloning strain. I succeeded in getting DpnI expression using this strain (albeit in smaller quantities I would normally get by using BL21). A better strain, which is JM110, is available around (https://www.addgene.org/49763/) that is more suitable for protein expression.
After this, you either can use a DE3 lysogenization kit to integrate DE3 module (which has T7 pol under Lac promoter control) so that you can utilize T7-based systems, or use a E. coli promoter. What I did was to clone DpnI after the tac promoter (a chimera of trp and lac promoters) to do expression.