OK Jack - I'm not an expert, but can answer most of your questions.
1. There is no need for a pressure fitting as gas engine carbs have a pump - this also allows you to place the gas tank where you want without regard to the carb height.
2. The 2 cycle oil is more like that used for chain saws, weed eaters, and best of all 2cycle motorcycles. The mixture ratio is important and will be recommended by the manufacturer of your engine. Most lawn mowers today are 4 cycle and use regular motor oil.
3. The fuel tank itself is not really any different although usually larger. The fuel lines (including the pickup line inside the tank) need to be of a type made for gas and the stopper on some brands of tanks needs to be changed.
4. The grade of gas depends on your engine brand - usually the higher compression types prefer premium and the lower compression types run better on regular. The instructions included with your engine will recommend the correct mixture and fuel grade.