All electric vehicles complying to the SAE J1772 standard are able to charge from 120V/208V/240V 60Hz single phase supplies. Almost all mass market vehicles (excluding DIY EV's) will be compatible with the J1772 standard. With that in mind, a quick background to the standard will be helpful to this discussion. J1772 is simply a means of connecting onboard chargers within the vehicle to a supply while minimizing the risk of electric shock and circuit overload without the need of the user to intervene. J1772 mitigates electric shock through a "make before break" pins connection strategy, a proximity switch which is depressed when the user wants to release the plug from the vehicle's receptacle, and through use of a ground fault interruption protection scheme set a bit higher than a Class A protection device. The J1772 EVSE has a contactor inside which does not close until it hears from the vehicle stating it is okay to start. The overload protection is accomplished through a square wave with variable duty cycle, which "advertises" the maximum allowable current to be drawn. This is always 80% of the OCPD feeding the EVSE. The vehicle then reads this value, and per the standard, does not exceed this value. This value can be higher than what the vehicle is capable of drawing. Think of the J1772 EVSE (what some refer to as a charging station) as a smart extension cord.
The standard allows up to 16A to be drawn at the 120V, or Level I, voltage level. Almost all vehicles will only be able to draw 12A from a Level I source. All vehicles sold recently come with a Level I EVSE that is plug connected to a NEMA 5-15P. This allows for charging away from dedicated a EVSE. This will draw a constant 12A from a 5-15R. If using an existing outlet, the user must be wary of overloading the circuit if it is not a dedicated circuit. As a side note, EVSE's do not exceed any safe level of current draw on outlets, but if there is any deficiency in the wiring, a constant 12A load on a standard 15A or 20A circuit will make that clear pretty quickly. Old, loose receptacles and loose connections can quickly heat up and become an issue. Be sure to use a new, quality receptacle when dedicating an outlet to EV charging.
Moving on to 240V, or Level II, charging. These can be plug connected or hard-wired. The ampacity scheme is the same. The EVSE advertises the maximum allowable current, and the vehicle complies. The key difference here is the maximum the vehicle will be able to draw. For example, a common EVSE setup is a Level II EVSE on a 40A double pole breaker. The EVSE will advertise 32A to the vehicle. If a vehicle with a capable onboard charger plugs in, it will draw the full 32A. However, if a vehicle with a lower capacity onboard charger plugs in, for example the Chevy Volt, it's maximum charger current is 15A at 240V. So, even though it is being told it can draw 32A, it will only draw what it can. Level II charging can range anywhere from 6A (which is pretty uncommon) to 80A. The sweet spot for the majority of plug in vehicles would be the 32A level. However, if capacity is an issue, a 12A Level II EVSE will still supply twice the power as a 12A Level I EVSE, cutting charging time in half.
The home is where the majority of users will be charging, and from a design standpoint you want to get them as much power as you can without overloading your service. However, with what you stated, you probably do not have that much capacity left in your system. A bare bones basic approach would be to provide a 15A, dedicated 5-15R GFCI outlet for charging. This will get the job done, but if residents drive more than a dozen or so miles a day, they will be left without a full charge. A 20A outlet will only be useful to residents with a Tesla Model S or Roadster. All other vehicles can only charge at 12A at the 120V level, so think twice before using up more of your load calculation to give 20A circuits and 5-20R's. Now, moving to 240V will not reduce the demand by your users. The minimum you'll be able to install will be a 12A Level II EVSE. If you can do 12A Level II, I would try to accommodate it, simply for the reason that charging 12A at 120V can take upwards of 15 hours for a full charge. Another design consideration is wire gauge. These devices will be drawing the 80% capacity for upwards of 3 hours, possibly to over 24 hours. Voltage drop can be an issue if the circuits are long. I would suggest at least going to the next gauge up to combat this issue. Lower voltage simply means less power for the vehicle's charger, which will not exceed the advertised current level; so more time charging.
So, in summary, you have the option to install whatever combination you like in terms of voltage level and ampacity. You have to figure how many spaces you want to serve and what that will mean for your service capacity and load calculations.