Second child needs the toddler bed, WWYD?

I have a little girl, 3 years old last December, and a little boy just turned 18 months. My youngest is an absolute monkey, and is a hair from climbing put the cot. I need to move him to the toddler bed, but my daughter sleeps so beautifully in it, and is so attached to it, I'm anxious about moving her out of it.

I have two choice, either I buy a second toddler bed for him, or I buy a single for her and transition them both.

I'm hesitant about buying a new toddler bed. My 3 year old is very tall, I'm not sure if she'll last in her toddler bed until 5, so we might only need two toddler beds for a year. I like the John Lewis Elementary bed, but frame and mattress still cost £200ish. It's not so much the money as the waste of it!

I'm also hesitant about putting her in a big bed. She's ASD and quite sensitive to change. I'm not sure I can handle two shit sleepers at once. She also looks cosy and happy in her bed! Plus all the single beds I can see recommend 4+.

That said, it does give us more longevity. I'm not sure what the set up will look like in terms of room sharing or bunks in the next few years. We would love them to share a room because my son's current room is TINY and putting them both in the bigger room make sense. My daughter loves sharing a room with her cousins, but I'm not hanging my hat on that working out every day.

I think I'm probably going to just buy a second toddler bed but I'm interested in what the hive mind thinks.