If you are using software then why not start with something like this http://www.thomann.de/gb/alesis_trigger_io.htm
(only trigger to
midi/usb no internal sounds ) then combine it with some pads (could look 2nd hand for
You can then make sure that you are getting the pads that you
want. If you can try out the pads first then you will know if they are right for you.