HO is a standard scale, as well as the most common scale by far.
Is all HO track interchangeable? Well, sort of. Standard-gauge HO track is all the same width, but the rails come in several sizes (called codes)--the most common is Code 100, then Code 83. They can be used together but require little adapters.
Recently, HO track has come out that is made with built-in plastic roadbed, rather than just the ties and rails. The plastic-roadbed kind is not interchangeable with other brands of the plastic roadbed, but most can be made to work with the non-roadbed kind.
There are two kinds of couplers--the cheap kind (called "horn-hook" couplers or X2Fs) and knuckle couplers (known by many names and brands, the best known is Kadee.) It's easy to swap out knuckle couplers for Kadees if you want to later.
In general, though, model railroad equipment has a surprisingly good set of standards. There are a few exceptions, but generally equipment is made to work to a common standard.
It's probably worthwhile to spend a bit more for a better-quality set, if only because it is less likely to break after a few hours, derail frequently, and look like a cheap toy.
