Hi and welcome!
No experience with Waldorf, so I can't give you any specific advice on the transition from Waldorf to Montessori... but I thought I'd reply anyway, mostly just to say hi and welcome you to the Montessori forum.
If you haven't already come across it, I recommend the book How To Raise An Amazing Child the Montessori Way as a very accessible introduction to Montessori. It includes a lot of ideas of things you can do at home plus a short section on what to look for in a Montessori preschool/school.
There have also been quite a few threads here about "what to look for in a Montessori school" so it's definitely worth doing a search to get a better idea of what to look out for. We had a choice of two Montessori preschools for my son - one in our village and one in a neighbouring village (12 minute drive each way). We ended up going with the preschool in the neighbouring village... I don't think there was really a single deciding factor - I think it just made a better overall impression - the staff were friendly and knowledgeable, a lot of thought had clearly been put into the environment, kids seemed calm and happy, great outdoor play area, etc. It was also accrediated by Swedish Montessori association (yes, we live in Sweden) which the other one wasn't.
We have been really happy with our choice. One of the things that has impressed us the most has been the effort the staff have made to *really* get to know my son - his preferences, unique talents, sense of humor, etc. etc. The have always treated him very much as an individual which has meant a great deal.
Anyway, very best of luck with your preschool hunt! Really hope you find something that suits your family.