I've operated pins for a few years now. It's not a big money-making operation, but it does turn a small profit.
Regarding that commercial lease, try to get at least fixed rent increases and the right to renew for a few years. Once you are operating and have customers who know where you are it becomes harder to move - and I've seen rent increases kill businesses all too often.
The more pins you have, the more parts you're going to need on hand. You don't want to wait a week waiting for a new $3 plunger (plus $8 shipping, ouch). The number of things you will want to keep on hand adds up fast, begins to take up a lot of space, and requires many storage bins/systems to manage it all.
Tools are a significant cost as well.
Also the more variety of pins you have, the more your parts inventory increases... Meaning Gottlieb, Stern, Bally/Williams... they all require different parts, and between eras of the same manufacturer they require different parts, too.