Hi Bogus,
Ok, some stuff for you to consider. ALL STBs pretty much do the same thing - show a nice widescreen digital picture on the TV. They don't really come with lots of features. The chanel changing speed is a definate factor but an expensive model is just as likely to be slower than a cheap model. About 1 second is as fast as it gets.
The main 'annoyance' of a STB is that it doesn't intereact with analoge appliances well (ie VCRs and DVD Recorders). Essentially if you want to send your signal to be recorded you have to make sure each box is tuned to the same channel. So a STB with timers can be handy in this situation (still a hassle though)
The main variation with STBs is the number/type of outputs. HiDef STBs havemore options and usually have the new digital only connections like HDMI, DVI etc. However a standard component connection is pretty much as good as any of these to 99% of viewers.
HiDef is pricey n the STB area, but recently got a bit more sensible. You can by a decent HiDef STB for under $300 now days. A SD one for around $130.
As for China -
forget it - different broadcasting setup to Oz (as is pretty much the rest of the world) so you're just as likely to bring back a unit that shows a nice hidef black screen

Note prices in this area are unlikely to be very much cheaper overseas anyway.
Personally I'd suggest you look into getting a PVR (A STB with a hard drive) you can pause live TV, skip ads, record digital TV effortlessly, and much more. Prices start at $250 and head up to $800 for top of the line SD models. There is only one HD model at the moment (The J35) and its not very well regarded (too many bugs) and it costs over $1000.
Some people get a HD STB for watching the occassional movie in HiDef and a cheap SD PVR (Like the Wintal) for recording and watching stuff they don't watch live. Personally I have a HiDef TV but only watch it via my twin tuner Topfield SD PVR. The joy that a PVR brings to watching TV (no ads, 7 day EPG, etc.) far outweighs the slight improvement in PQ I see in HiDef. Eventually I'll get a HiDef PVR but till then I'm very happy with SD PQ (esp as most of whats broadcast on HD TV is really just upconverted SD).
Note that only one model of PVR/STB can have a 7 day EPG - The Topfield - all other boxes can usually display an EPG but the networks don't broadcast the data they need.
Regards
Peter Gillespie
PS Do a seach on Toppy, Humax Smart, All Questions Answered, Wintal, etc for some more PVR info.
PPS When you say your TV can 'handle HiDef' do you mean 'HiDef Ready'? A lot of TVs are marketed as being able to show Hidef but what they really mean is the TV down converts the HiDef picture and shows it as SD. In this case getting a HD STB would probably be a waste of money. What model TV do you have?