When to move DS to a bed?

[Treesandbees]

We have an 18mo DS whose in a cot bed. Currently sleeping great and showing no signs of climbing out! We're expecting DC2 in summer and need to move DS into spare room as its at the quite end of the house. Do we buy another cot bed and then convert to a bed when he's ready or just go for a toddler bed or even a single bed? He will be about 20 months when we plan to move him.

My eldest two both went into a bed at around 18 months

Youngest is now 21 months and I've started looking at baby proofing her room in preparation (have been using one side as storage)

Oh forgot to add

Never had any problems with older two in beds before 2 years

We bought a second cot bed with a 2 yr gap.
DS1 had the box room. We took the sides off the cot when he climbed out about 25 months. he stayed in the toddler bed til we bought a cabin bed aged 5 ish.

DS2 climbed out of the cot about 18 months. He is still in the toddler bed approaching 4.

Thank you! I think a cotbed is a good compromise as he's showing no signs of climbing yet so would want to keep the bars up as long as poss!