Hi R., I was in the same position except it was my son transitioning. We bought him a full size bed (brought him to the store and everything). Then we set it up in his room in the corner with his crib right next to it (on the side). He slept in the crib for a few nights and then started requesting the bed for naps and then for bedtime. Also keep in mind we had the baby in the room with him (due to moving to a new place when the baby was 3 weeks and the fact that my older son never had his own room before). It worked out great. Oh, but the baby was in a playpen bassinet.
The great thing about the full size bed is we could put his stuffed animals in it, some books, a toy, and he still had plenty of room to sleep and the feeling of a crib with the crib on one side and the walls on the other 2 sides.
Another side note is that due to odd architecture (and it being a rental) we put the stair safety gate on their bedroom door. I never had a problem with him getting out of bed I just had a rule that he could bring his toys to bed. He would play a few minutes and then lay down and pass out. He didn't bother the baby, but maybe we were lucky and we did have sound machine. Also our crib is a sleigh bed, so my son couldn't climb in it from his bed due to the sleigh side being against his bed.
Side note on toddler beds, in my friends experiences they were either used very well, or not all, no real in between.
good luck!