The higher the ABEC number, the tighter the manufacturing tolerances are held. Smaller wheels are slower, and are used more for street or trick skating, while larger wheels are faster and are used more for recreational to speed skating. Get bigger wheels if you want to go faster, smaller if you want to turn more, do tricks etc. Softer wheels will have average wear, high traction and a smoother ride while hard wheels will have longer wear, lower traction and a rougher ride. Bearings will get dirty from exposure to water, dust and other elements and should be cleaned.
Tecnica bearings are fully serviceable.
Remove the bearings from the wheels and remove the shields from the bearings. Wash all dirt and grease from the bearing with a solvent and toothbrush. Allow the bearing to dry. Add new lubricant and reassemble the bearing. Your local retailer can either do this for you, or provide the necessary supplies.