I agree that visiting and testing if you have the luxury of time are essential. I thought Montessori would be the perfect fit for a gifted kid too -because of them learnng at their own pace. It turned out the school we used actually held my child back from things he could do - saying he couldn't do them until the following year! That didn't even follow Montessori principles. A couple of years into the school it became clear that temprement is also highly important. Our child wanted to play and socialize and had mostly been around adults. There was also no system for discipline in place that worked with our child's temprement. How the school is set up to deal with problems is really important and not something you think about when you child is 2, has not had time to be naughty and isn't a trouble maker yet. But if they become a trouble maker - then knowing how the school deals with discipline is important. I found schools reluctant to talk about this - they wanted to behave during initial interviews like all kids were "perfect" and not be negative - and as the parent - you hardly want to walk in and say "my kid might turn out to be a trouble maker - in that situation - what would you do?"
A county counsellor helped us identify our child as highly spirited. I recently read an article that Montessori often does not work well for super shy or super spirited kids - and in my experience that was the case. But how each school is being run is of huge importance and I didn't have the luxury of day time visits or the time to go around several schools. Luckily, I had a stay-at-home mom friend that visited a ton of pre-schools on behalf of both our children and she helped us find a church school in the end to provide more structure.
Montessori philosophy can differ depending on who runs the school, how accredited they are, and whether they are accredited with the American, or World/International Montessori Associatons. (There are several). The school has to meet certain requirements to get these accreditations. Note though: they don't always have to meet every single requirement or even get every single Montessori accreditation in order to open and legally function as a Pre-School (as was the case at our school).
Any Pre-school is a scary decision when it's your first - so I totally undretand what you're going through emotionally. Don't rush - espeically as you do have the ability to stay home and watch your child and especially as you don't sound like you'll be losing a huge deposit, fees or a place at the school you want. Often the waiting lists alone at most preschools start signing kids up as early as March of the preceeding year - and in my experience - there was pressure to renew our contract with schools as early as March - which seemed ridiculous when a child of a young age just started i September and when a children develop so much in a few months at that age. It's hard to predict what they need and hard not to feel pressured about losing your place at a current school. Demand for Montessori can be really high - so best to start the process early and learn as much about that type of school from the various Montessori websites.