It's like buying a car - you can buy a second hand one & have no problems with it, or you can find yourself stuck with a lemon. Same with buying a new car - it will work fine, it might have a few niggly problems, or it might be a lemon.
I bought my HX4700 brand new & it's not had any problems so far (touch wood

); but if you look through this forum there are other users who have had problems.
Most faults will rear their ugly little heads at the start of the unit's life, so if it's a few months old & working OK then you can (usually) assume that the unit will continue to behave itself.
As for incomplete/complete packages, you really need the cradle, a power lead & the software CD - the manual can be downloaded & the box isn't necessary. If you have at least the CD then you can price the other items & see if it's worth buying the unit & then buy the other bits that you need, or decide instead to buy new & get everything you need in one box.
Have a browse on the internet & see if there are any bargains to be picked up for new units - some providers put together bundles (e.g. to include a screen protector, case etc).
But the final decision of whether to buy new or old has to be yours.