Used mini split installation

[SDMCF]

I recently bought a job lot of used stuff that included 2 mini split systems. I know little about such systems and I would welcome any advice about how to install these - and indeed whether I should install them. I have read a lot on here, especially the "Mini Split install" thread which was very helpful (thanks) but I am unclear about how installing used items might differ from installing new equipment.

The units I have are Mitsubishi MUZ-FD25VABH (outside) and MSZ-FD25 (inside). I know where these came from and I am confident they were working well before removal, but I did not remove them myself and do not know how the removal was done. So I don't know if the regrigerant has been lost, or what is currently in the devices. Nor do I know how to check that. Does this mean I should remove the content of the equipment and re-charge from scratch? If so, how do I do that and what do I do with the material I remove? How do I re-charge the units?

Our climate is such that I would rarely use the devices for cooling, so I am only really concerned with the heating capability. My doubt about whether I should install the units concerns how efficient they can be compared to my existing heating, which uses a ground source heat pump. At a simplistic level I would assume that the efficiency of my GSHP is similar to these mini splits, so the main factor is the temperature of the heat source (ground or air). For most of the winter the ground will be considerably warmer than the air. Does this mean the splits would only be more efficient during short intervals at the start and end of the heating system when the air is warmer than the ground? So is it worth fitting them or would I be better off re-selling them?