You say that each axis has one proximity switch at one end of each axis. This implies that the switch is there for homing the axis. Depending upon the software used to drive the machine it should be possible to rearrange the switch to act as both homing and limit switch. Re-locate the switch to the side of the axis and arrange sensing blocks to protrude so that they pass the sensing face of the switch. One block at each end of travel. One neat way is to arrange for fixed sensing blocks to limit the travel and movable blocks to reduce the travel for small jobs. This saves the machine travel to the end of axis to home. However please note that these switches are not normally designed to act as mechanical stops despite their apparent robust construction.
The use of electronic proximity switches is begining to extend down to more modest machines as the cost has reduced a lot over the last couple of years. Currently £6GBP will get you a good quality electronic switch where as a sealed microswitch with a roller lever is the same price and a fully encased model arround £30GBP for the microswitch and housing. If properly set up the accuracy and reliability will exceed that of the mechanical switch. Depending upon the type of electronic switch - optical - magnetic - inductive - capacitative - the switch will offer advantages over those of a mechanical switch in addition to being fully waterproof.
If the new machine is fitted with servos not steppers and has no electronic protection for motor overload then it migh be worth considering safety switches at the end of travel just before the axis hits the buffers. This would help protect the motors from burn out. Not necessary for steppers.
Hope this sets your mind at rest and you enjoy your new machine to the full.