Dumb laser distance measurement should solve that easily for you. You can literally use some very basic math with thruster output based on your downward distance/speed.
What happens when you over-compensate with the thruster?
Sometimes you can recover from that, sometimes not.
Managing delta-v is half of what makes a Moon landing require precision. An atmosphere can let you ignore this half to a degree (depending on specifics of vehicle/mission).
Anyway, the easiest way to get a feel for these things is to play some Kerbal Space Program. Try doing some moon flights. You'll quickly realize that the part of the mission that feels easiest on the home planet (landing) also feels the hardest on atmosphere-less bodies, and vice versa (e.g. taking off, or slowing for landing).