Do you mean adjustable dumbbells, where you attach different weight plates? Those are fine, albeit a bit bulky, and 2.5kg is a good starting weight.
I started off with 3kg, then 5, 8, 10, 12.5. I bought individual dumbbell sets rather than an adjustable set because I find them easier to handle. Bought over a period of about 3 years it’s not expensive, and I still use all of them for different exercises, except the 3kg (which found a good home with a friend).
Someone I know got a set of dumbbells via Freecycle’s ‘Wanted’.