Whatever you get, here are a couple of points to ponder:
1) You will either need a very reliable and available service technician that has all parts on hand for your machine -AND/OR- you need to have a backup machine and the ability to quickly repair it yourself. Downtime can kill you in a situation like this, and if you get some fancy newfangled machine that does not have interchangeable parts (with lots of availability) you can be down for days if something fails. Even if you get a well-known brand name, you will find that each manufacturer has some oddball sizes and setups (look at Simonelli's parts catalog and you'll see what I mean). The ability to quickly replace a failed pump, solenoid, or valve can make or break your bottom line...
2) As Stubbie said, if you only have one worker, it is pointless to have more than 2 groups. Even if you have 2 folks working, one barista and one cashier, 2 groups will be plenty. The thing that I see frequently is guys with 3, 4, or 5 groups that never use more than 2 of them.
3) A goal of catching 2% of your potential customers is probably quite reasonable for a grocery or department store. You'll probably see more like 0.3% to 1% until you are well-established. With that in mind, and 30k potential customers a week, that is probably about 7k a day on the busiest days (30k weekly probably means about half the business happening in under a quarter of that time). So you are looking at serving something like 75-150 espresso beverages on good days. That is still well within the 2-group/1 barista zone...
My estimates above are based on a fairly saturated West Coast market - where most people will drive past several decent espresso opportunities on their way to the store. Having visited other parts of the country where this is not the case, you may find yourself 'catching' more of the demographic...
FWIW- I have seen guys with $1200 third-hand Faema 2-group machines burn through nearly 100 pounds (of really great coffee) on a good week and make bank. I have also seen guys with $15,000 Marzoccos lose money more weeks than not unless they pretty much lived behind the machine themselves...
One more tip - the guy I know who did the best (until he sold out and retired) usually had young women from his church working for him.
