A tip when looking for tips, have a look at the ones Thermaltronics (a company founded by ex Metcal employees) make. These tend to be a bit cheaper, and are basically clones of the Metcal sizes. They have a cross reference chart.
Another thing to look for is the tip temperature, there are 600,700 and 800F tips. I have the 600F ones, they should be fine for most needs (maybe not the best for lead free). Lower temps are usually better, and it makes the flux not burn and blacken on the tip as severely.
As for tip sizes, having a couple is never bad. Right now I use the M6C300 for smd stuff (tiny conical), the MCH178 for "small" through hole (1,5mm chisel), then there is the M6CP200 for big connectors/wires etc (3.0mm chisel), and finally the M6CP200 hoof tip for drag soldering. An advantage of the Metcals heating architecture makes it possible to often use somewhat undersized tips, the feedback is so quick that it still manages to heat the thermal mas well enough to get the job done.