It would probably be a good idea to do some preliminary research
on a particular model or a subject (say T-34/85) you want to
purchase and build. Usually, I would not buy a model unless I
can find one or two favorable reviews on it. Of course, this
does not apply to really cheap sales.
Generally, recently made (but not reissued!) models of all major
manufacturers are excellent in quality but pretty expensive. With
older and cheaper models research becomes essential: many of
these models are worthless junk, while some are great even up
to the modern standards or can be turned into very decent models
by adding some photoetch, nice tracks, etc.
You can usually roughly estimate how new a model is by looking
at it's catalog number: the higher the number the better. If you can
examine the box (which, of course, only applies to offline shopping),
you will usually see the year (next to the copyright sign) on it.
Hope this helps,
Serge