I use kettlebells a little bit with my clients for general conditioning work.
I've got,
4kg x2
8kg x2
12kg x2
16kg
20kg
which cost me $250, and that's been plenty for the people I train, who are almost all beginners.
You can do a routine of,
Swings 1'00" - max reps in that time
Goblet squats 1'00"
Rows 1'00" - get in the "starter runner" position, supporting hand on supporting leg
Overhead presses 1'00"
Rest 1'00"
repeat
Start with the 4kg for a warmup, in the next set use the 8kg, then the 4kg for a cooldown. Smaller people can use the 8kg two-handed for swings and/or presses, larger people will do it one-handed. Next fortnight, add another work set with the 8kg. The following fortnight, a third work set.
You need someone standing nearby to count it out with a stopwatch, if you don't have that, just do a certain amount of reps, eg
Swings 1x10
Goblet squats 1x10
Overhead presses 1x10
next week do 1x12, following week 1x14, and so on up to 1x20.
and a single 4kg and 8kg will be plenty, most beginners will struggle to put the 12kg overhead very many times after doing the other stuff at a decent pace.
Once the person can do 4-5 sets of this at the work weight, they need either a heavier bell - 12, 16, 20kg. At this time they might also add snatches, though those are a bit technical compared to swings, squats and presses.
Of course all the sorts of isolation work you can do with dumbbells, like curls and front raises, you can also do with kettlebells. I prefer all compound movements, myself, at least for beginners.