Hi,
I've used a lot of different brands as we had a wide range in college when I was teaching level 3 facials.
Carlton and silhouette brands were both reliable and we could always get spare leads when needed.
What I would suggest is that when it comes to micro-current you are best to go for a system where you do the whole face with probes as this is 'proper' micro-current.
The machines where you stick pads with wires onto the face are actually using EMS (electro-muscular stimulation) so not true micro-current (although they do achieve muscle contraction) and your client feels the current.
In college we trained the students using the Goddess 2000 as this used pads on the lower face and probes on the upper face. This way we could assess them for EMS and micro-current all in one facial. It was actually a good treatment and the units weren't too expensive.
In my own salon, what do I use? Biotherapeutic Ultimate Gold (more recent versions available).
This system uses probes and micro-current for the full facial, has skin boosting programmes and you can use it for galvanic purposes if wished. The client does not feel any current. As always, there are far more programmes on it than I ever use!
Caci is another alternative you might want to look at for combined currents but an expensive option.
Guinot hydradermie (or whatever they call it now...) is good too but you'd need all their products too.
Second hand equipment - yes, I've done this without any problem for both my bio-therapeutic unit and my sterex blend electrolysis unit. I had to repair a button on the micro-current unit but even with paying for this it was still loads cheaper than buying a brand new one.
I purchased from David Howarth via ebay and he sells / services / mends units:
https://www.ebay.co.uk/str/complexionsonline/Used-Hair-Beauty-Equipment/_i.html?_storecat=732582016
Hope this is of help