Based on optical high‐resolution spectra obtained with CFHT/ESPaDOnS, we present new measurements of activity and magnetic field proxies of 442 low‐mass K5‐M7 dwarfs. The objects were analysed as potential targets to search for planetary‐mass companions with the new spectropolarimeter and high‐precision velocimeter, SPIRou. We have analysed their high‐resolution spectra in an homogeneous way: circular polarization, chromospheric features and Zeeman broadening of the FeH infrared line. The complex relationship between these activity indicators is analysed; while no strong connection is found between the large‐scale and small‐scale magnetic fields, the latter relates with the non‐thermal flux originating in the chromosphere. We then examine the relationship between various activity diagnostics and the optical radial‐velocity (RV) jitter available in the literature, especially for planet host stars. We use this to derive for all stars an activity merit function (higher for quieter stars) with the goal of identifying the most favourable stars, where the RV jitter is low enough for planet searches. We find that the main contributors to the RV jitter are the large‐scale magnetic field and the chromospheric non‐thermal emission. In addition, three stars (GJ 1289, GJ 793 and GJ 251) have been followed along their rotation using the spectropolarimetric mode, and we derive their magnetic topology. These very slow rotators are good representatives of future SPIRou targets. They are compared to other stars, where the magnetic topology is also known. The poloidal component of the magnetic field is predominent in all three stars.