The CNC router from China commonly referred to as 3018 Pro CNC Router is fairly inexpensive and can be upgraded fairly easily. This Thing will help you install limit switches which help prevent damaging the machine by crashing it.
This thing is intended to be used with the Pro version which is made of both extruded aluminum and CNC routed plastic parts. The standard version is mostly made of extruded aluminum. While this might work with both, I do not have a standard version to test it with.
Qty 2 - X-Axis Limit Switch Holder V2.STL
Qty 2 - Y-Axis Limit Switch Holder V2.STL
Qty 8 - T-Nut V2.SLDPRT
Qty 4 - Limit Switch model: V-153-1C25
Micro Limit Switch Long Straight Hinge Lever Arm SPDT Snap Action
You can search AliExpress for "V-153-1C25" just make sure you get one with a long arm as specified by the model number. The long arm is is needed for the X Axis.
Note: the Y axis will work with a medium style arm, but the long one works fine.
Qty 16 - M3 nut
Qty 4 - M3x12mm screw
Qty 8 - M3x16mm screw
Qty 4 - M3x20mm screw
Here is an example of a kit which includes the the hardware you will need. You could get much cheaper hardware if you take the time to look for it (and maybe wait for it to ship from China)
You can find this router all over the place, eBay, Amazon, AliExpress, Banggood, etc..
Search these sites for 3018 Pro CNC Router
Here are a couple example links.
Note: the SainSmart site shows various models of 3018 router including the Pro and non-Pro version.
The main differences between the different 3018 Pro CNC Router versions you find online is the inclusion or exclusion of options (range: $100 to $300USD):