One of the major disadvantages you'll have to figure out how to deal with is that a pillar - if not the core itself - of Montessori is teaching others. You have two children, so you can teach this to some extent; but you're going to have to either get together with other like-minded folks, or else figure out other ways to get those benefits, particularly for your younger child. It's possible to do, but you will have to think about it; it's something I would make sure to have a good solution to at the outset.
You also will face a significant expense in purchasing materials, if you're going to follow all of the practices; Montessori is very hands-on, and so while a "regular" math class would involve just pencil and paper, a Montessori class uses bead strings, number tiles, and other similar physical and visual aids to help the children understand the concepts at a more physical level. The materials for a preschool to kindergarten (3-6) class are going to run several thousand (US) dollars, just one set of everything (which should be fine), and then additional purchases will be required as they get older. You may be able to find some (or even all) of the materials used for less, but it will not be a small expense.
Both of these issues are less problematic if you can find other like-minded parents and work with them. You can share these expenses for purchasing supplies (you don't need all supplies every day of course), and you can have days where the children work together to gain the advantages from that. That would always be recommended for home schooling, but in particular for Montessori I think it's necessary.
As such, I would strongly recommend finding out if you can find other parents in your area. If you're in (or in a suburb of) a larger city, there are certainly homeschooling networks; some of them will likely be Montessori-influenced.
I'd also suggest talking to some of those parents you find and perhaps observing them if they'll allow it. Homeschooling requires you to do quite a lot of work, and requires a good understanding of what's needed to help your child grow. Watching another parent who's successfully doing so will help you get an idea for how you can approach things - and this will continue to be true even as you become the parent who is successful, as you learn yourself.
Finally, there are some excellent online resources for Montessori education. Montessori.edu has a page on homeschooling, for example; several sites have pages about setting up a Montessori home school environment. There are quite a lot of pages like this out there; look around and find the one that matches your needs.
As far as whether it's too late to start, it's not; 5 is later than usual, perhaps, but if you've been teaching your child to be self-sufficient and to do things like put away his toys and clean up his plate, you've already been teaching him things that he'd learn in the preschool portion of Montessori education.