What is your price range?
Rough Guide::
0-£20 will get you something grav fed, like a Proto Primo, or a cheap agitation like a Viewloader egg
£20-£40 will get you something better, usually sound sensitive, like a Halo v35
£40-£50 will get you something in the Prophecy range, basically a better Halo.
£50-£100 will get you either a Prophecy z2, or a Rotor. (Z2 is better in every way, aside from its eating of batteries)
£100+ will get you a spire.
Other things to consider are whether you want a speedfeed and feed rate.
For example dont stick something lower than a Halo on it and wonder why you can't keep up with everyone elses fire rate.
For a Mini, and casual play I would advise a Halo upwards. Agitation hoppers just wont get you any consistent shots above 10bps