there's really only two things to worry about when it comes to an ebay kit (and well...really, any turbo kit)- the manifold and the turbo.
to the best of my knowledge, the log manifold usually doesn't crack due to the simplicity of its casting, but ebay ones do have a reputation for cracking all the time- but at least they're cheap to replace. same thing for the turbo...many reports of seizing and compressors failing pretty soon.
by the same token though, many ppl have been lucky with their setups and run them for years without trouble....
so if you want to save money...throw your dice and keep your fingers crossed