I've fitted a few to our cars. The mechanical side is easy, chassis usually pre-drilled & it is just a case of bolting the thing together. Might have to lower exhaust, some need a bumper cut. Put one on our Mondeo last year, probably as easy as it gets. Took less than an hour to attach the bar, another hour or so for electrics. Mfrs dedicated wiring kits are nice but pricey. Hardest part usually getting behind trim panels. Running a lead from the battery if your fitting a Smart By-Pass relay can be a pain.
I'd be wary of s/h unless your certain all the bits are included & undamaged. Always a chance something could get left on the donor car.
We tow a fairly hefty caravan so I'm pretty anal in checking bolts are torqued correctly etc. I'm pretty sure they have to inspect the tow-bar on MOT's these days, (if fitted obviously).