To fully charge a 12 v battery you need about 14.4 v. If it is a wet battery then you will need to equalise it occasionally to near 15v so your figure of 18v is reasonable.
Bear in mind that the actual battery voltage is temperature dependent and the panels are even more so. If your panel can make 18v at 20degC it will not make this at the temperatures in full sun on a hot day.
Most 12v cells have the peak power point about 17v under working conditions, most make about 21v off load in cold conditions.
There is no limit on input voltage with a current source such as solar, the battery will determine the volts and the panels determine the current. With a mppt controller you needn't worry, high voltage panels will be matched to give full power with an increase in current.
Without mppt you are limited by the panel current ( virtually its short circuit current) so by using grossly over voltage panels you are throwing away useful power.
If you used 2 x 12v panels in series it would do the same thing as one. If you connected them in parallel it would give twice as much.
If you are direct connected and you want to use higher voltage panels you can do so but with a loss of power compared with their rated output at correct voltage.
Regards panels as a current source, it's their short circuit amps figure on the label that gives you most idea. You will get close to it if matched reasonably.
The wattage rating doesn't mean much, you will probably never see it as the insolation figures are high for the makers test and the test is at a panel temperature of 25C. If you get the insolation figure it will raise your panel temperature above 25C. You may see close to it on a bright cold frosty day.
Flux
