Few tips... I have one and still can’t use it
If it’s industrial it’ll have a motor the size of a keg and anything else is domestic.
Motors come in 2 forms
Clutch - noisy expensive to run but the speed is easily variable, but you have to feather the accelerator as they gently gain speed.
Servo - silent modern motors better torque, but the speed is varied by a switch but is configurable.
They also come in 2 different volt strengths (I’m not great on this bit) 240 or below or 500 500 requires industrial strength power.
240 is home based power mine is 500 so I can’t use it without it blowing the power! So I’m now looking for a 240!
About the actual machine you need to be very clear about what you want your machine for, because industrial machines do 1 thing or a limited number of things really well! Work out your requirements and find the right class of machine.
For example:
95k does straight stitching at up to 3500 stitches a minute and does light medium and heavy fabrics maybe light leathers.
20u Singer does light medium and heavy and unusually has a straight and zigzag and the zigzag goes to 12mm (from memory)
Bernina 450? Is an industrial with a lot of embroidery stitches probably light or medium fabrics.
There’s a multitude of various leather grade machines, with different walking foot options the most rustic is a wheel through to modern multipart feet there’s some good leather sewing forums out there with excellent advice.
Finally: needles
Check you can get them easily and they’re not extinct! Ie 29k needles for that are a quid each and when they’re gone there’s no more
Also consider harp size ie 95k has a space on a par or slightly bigger than a 201k but a 96k is much bigger!
I went for a fridge cabinet 95k because they’re compact, tough, handle the fabrics I need ie light through to light leathers.
Hope that helps
R.