Best for the money is the Integy Eagle MOD lathe. I got mine for about $90 bucks shipped from R&L Hobbies. It has the carbide V-Blocks which are the latest and greatest to have on your comm lathe and can use a bit for either side. Also the carbide bit that comes with it seems to cut for a long time.
Don't get a diamond bit, they can chip VERY easily and you will just cut comms out of round and not know what the problem is as it will still have that great finish. When a carbide bit gets dull, it will make the finish look dull. You can always just resharpen the bit or get a new one as they are really cheap. go to www.bigjimracing.com
to in the tips section and read about carbide vs diamond.