TIG parts are bought as a set, because the collet, the collet body and the cup have to come from one system. A gas lens needs a different body from a plain collet, so switching to a lens means changing two items, not one.
A gas lens straightens the argon stream and lets you extend the electrode beyond the cup without losing the shield. On stainless steel, titanium and in corners you see it at once: less discolouration, a calmer pool and lower gas use for the same protection.
Match the tungsten to current and material. WL15 gold and WC20 grey work well on DC for steel and stainless steel, WL20 blue is universal, WP green and WS suit AC on aluminium. Grind along the axis of the electrode, at 30 degrees for low currents and 60 degrees for high ones. Cross grinding marks scatter the arc.
Ceramic cups are marked with a number matching the bore in units of 1.6 mm. A number 6 cup is about 9.5 mm, a number 8 about 12.5 mm. A larger cup with a lens gives a better shield at higher currents, a smaller one lets you see into tight places. Quartz cups let you watch the electrode and the cooling zone, which helps with titanium.