Just like buying fresh roasted gourmet coffee beans to use, the espresso machine is the other Mack daddy of your whole coffee business. It is your life blood so in other words: DO NOT SKIMP ON IT! However, having said that when you are opening a coffee shop there is the line of overkill you do not need to cross either and that depends on your anticipated sales volume. I say, two group maximum, if you need more power or want a backup, get a one group as well.
The feasibility of a three or four group is great but it's difficult to get more than one person working on them due to spacing of the group heads, etc. Ordinarily, you do not need more than one person pulling shots and making the espresso beverages anyway. It is almost impossible for one barista to use all four groups efficiently at one time so you be the judge! However that may be up to debate if you get REALLY busy. However, a two group is always my choice.
Semi-Automatic, Automatic or Super Automatic- Well my choice is always the automatic because you can program them to cut off a shot at 23 seconds, or whatever you choose but still do it manually. A semi-automatic requires manual shut off by the operator and an automatic can allow for a programmed (timed) shot duration or allow the operator to perform it manually.
Super Automatic - This machine will grind the beans, tamp, pull the shot, shut it off and even discard the used grounds. Yes, I am serious. Most *$ and some indies are using these to get bad coffee through their cattle call rushes. I believe you lose a lot of 'art' when you use one of these machines. They are also big bucks (like close to $10k) and quite costly to repair. If you have to send one for repair, you have just sent your grinder too so think twice.
Boiler capacity - large enough for a big rush, 9-14 liters should be sufficient. You cannot run out of steam or water in a rush and with a smaller boiler that will happen! Trust me on this from experience!
If you are about to open a coffee shop you can get a used two group for back up just in case if you can afford it. If your new machine goes down, you can get service out rather quick and most likely a replacement for the time yours is being fixed. But you might be down a few hours or at worst, a day or two till you can get service. That is all lost revenue. The backup will be well worth it. It can also be used as an overflow in times of extreme business!
Just like deciding on a roaster that get you fresh roasted coffee beans quickly, you should try to buy an espresso machine based on the availability to get parts and service locally or at the very least quickly. Do not buy based on price alone, or 'coolness' or 'features' of a machine. They are all good these days. Features will not mean anything if you cannot get local service on your machine. Top machines, in my opinion are LaMarzocco and Nuovo Simonelli. I have three Simonellis.
Considering a double boiler? Espresso machines that have a double boiler have a small tank (actually it's more like a tube). It holds the water for the espresso shots and has another tank that holds the water to create the steam. This system is supposed to ensure that the water for the shots stays consistent in temperature, allowing you to create the best quality you can. It also ensures that there is always enough water available to create steam. LaMarzocco is the only machine that employs this system, and their price reflects it.
However, depending on whom you talk to, the double boiler system is up for debate. Some say it is not a sure thing to keep the temperature constant because the second boiler is so small. I have not noticed any differences as I have worked on both machines. Nuova Simonelli's and all the rest of the espresso machines use what is called a heat exchanger. That is, cold water is fed up to the heat exchanger and it is heated and ready for the group head to disperse it to pull a shot. I think they both work just as well. I say buy the machine that has what you need, not what you want but one that you know you can get parts and repairs for in the shortest amount of time.
When opening a coffee shop, hopefully these tips about buying an espresso machine will give you some insight as to what to look for.