On all the other kits, you have to buy the belts of your choice.
Yes, you want the crotch straps.
The crotch straps keeps the lap belt low, over your hips, in the case of an accident.
Otherwise the shoulder harness will let the lap belt move up, across your stomach, which will crush your vital organs. While you might survive the accident, you will die a painful death later due to your damaged organs that are now failing.
The way you install the belts is to put the crotch strap and lap belts on at the same time. Tighten the crotch strap to get it low then tighten the lap belts. Once that is done you tighten your shoulder harnesses last. (Most people tighten the shoulder harnesses first, which is wrong.