It depends where you are located and which coffee roasters you have available around you, lots of larger wholesale roasters can provide coffee, training and lease you machines at a very competitive price. That could be a good option if you want to serve espresso drinks for 50-100 customers and don’t have the budget to cover the costs upfront, although that depends on your specific needs. Otherwise as people have suggested filter batch brewers are also very good if you want to serve 50-100 customers.
Some of the cheaper commercial batch brewers don’t really produce very good coffee, generally the spray heads are of poor quality, so I would be wary. The moccamaster kbg 741 isn’t a bad option if you buy a second carafe so you can brew another batch when one runs low. Another option would be the Marco: BRU F60A which has a larger capacity and is a plumbed in option (the manual option is fine but requires more work).
You could either get pre ground coffee, or coffee from a local supplier and grind it yourself in something as cheap as a wilfa svart and get better results.
Coffee Roaster. Home: Sage Dual Boiler, Niche Zero, Ode v2 (SSP), 1zpresso ZP6 Work: Eagle One Prima EXP, mahlkonig e80s, Mazzer Philos and lots more