The least expensive way of owning a new mac is to go with the mac mini. Remember with the new mac mini, you cannot add more memory after buying. That forces you to max out the memory to 16gig when you buy. Remember to double any memory requirements you see as the minimum. The entry level mac mini is rather slow. You should go with a faster mac mini.
You could buy one system and load it up with all the software you will need and see how it works. I think you can load your software in an apple store and try it out there too. Just delete when done.
The cheapest way setting up your shop will be to go to ebay and buy all your software and hardware there. This puts a large burden on you to make knowledgeable purchases. You can save a bunch of money on software if you know that you can get by with an older version. You have to factor in your time. You want to be doing business not being an it guy.
You will save a lot of money by going with the mac over time. You will spend a lot less time on it administration.
You should think of customer impressions too.