Choosing the cabinet is the first step to get into building your modular.
The cabinet will influence what you will choose  for the following reasons:

• Compatibility
• Busspower
• Space to fill

Everybody that want to start with modulars and has a tight budget often choose to buy a small 1 row cabinet thinking to buy a new one latter when the first will be full, but that’s not really useful if you are trying to find the cheaper way.
If you have a little knowledge of bricolage you can do the cabinet yourself with the advantage of doing something that fits your needs as space and form.
Then just buy a Power buss kit from Doepfer (for example) and a couple of row to mount your modules and the case is done!
Another alterative is to buy an eurorack case from a electronic online shop that sell’s cases for electronic applications.
This are really cheap and rack-mountable , then you have only to buy the power buss kit.

If you don’t want to spend some time building the case yourself there are a great amount of cabinets friom the various manufaturers.
Doepfer Low Cost cabinets are great if you want somenthing “standalone” (not rack-mountable) and want to pay a quite low price.

Keep in mind that there are 3 main standards for power buss in euroracks :

• Doepfer
• Analog systems
• Analog solutions

Doepfer is the more used and compatible with all kind of euro products , but needs a conversion cable for ALL Analog Systems modules and some Analogue Solutions modules.
Analog systems has a different power buss configuration, and different hole screw positions, so if mounting a ASys modul on a Doepfer cabinet you will need a different cable to connect it to the buss and the module will be not aligned with the others, will ha a 1 empty hp on the left and on the right sides.
Analogue solutions is the same as Doepfer but for a couple of modules that require a particular voltage you will need a cable to connect the module to the buss.
So the main differences are between Doepfer and Asys.