If you're planning on buying beds anyway, I'd get a twin bed with toddler rails. It saves you money in the long run and will work just as well. My 2 year old is in a collapsed trundle bed and my 3 year old is in a regular bed.
Skip toddler bed, go for a full sized bed. Dd was 19m when DS was born, we went ahead and bought new for him cause we bought a bedroom suite for her along with the convertible crib... Did the same for him so we hopefully won't ever have to buy them furniture again. She only stayed in her toddler bed for a few months, she did so good in the full size bed and loves it! We switched her when she turned 2!
Jade was 22 months when I had her lil brother, we bought her the kura bunkbed from ikea, and he got her crib, she was sooo excited about the "big girl" bed she payed no attention to her crib in the new nursery
We skipped the toddler bed and got a bunk bed. My oldest was 16 months when his brother was born but we just put the bed rails on the bottom bunk and he loved it...thought he was big stuff. Saves money long run, skipping that toddler bed step.