Are you talking firearms, ammunition, or reloading equipment?
If I were looking to have a well rounded die setup so friends could come over and use my equipment and hang out, have a reloading party.
380
9mm
38spl/357 mag
40
45
233
308
You may run into some low volume hunting cartridges as well
44mag
243
30-30
30-06
45/70
I would skip 7.62x39 and 7.62x54R as most cases are steel and tend to be berdan primed, making them fairly impractical to reload for.
I would talk to your local friends / gun shops. probably only a few sell reloading supplies, and most can probably tell you the most common calibers.
Another thing to consider is powder, primers, bullets. Your friends will have to bring their own. You would have to help them research their loads as well before starting.