all depends on your budget mate.
best bet is get yourself a sequncer, high end ones like cubase will cost you around £250 -£450
the midrange ones like reason or orion platinum about £200.
there are allso some free or very cheep ones around.
personally i dont reason, i use cubase and orion but to be honest i prefer orion cus its so easy to use.
for those 303's there are a number of vst clones the best i guess would be audio realism about£100 there are some cheaper and free ones around allso.