If you want a brush with a replacement knot, there are a few ones where you can screw them in, but it's not my area of speciality. Or you could get a handle from ApShaveCo, and a few different knots and set them with bathroom silicone. If you want to remove the knot, all you have to twist the knot out, scrape off the remnants and put in a different knot.
I've not used a Muhle knot so I can't comment on how it performs.
What kind of brush are you after? Something which feels like a badger, something with scrub, a good all rounder or something which is as soft and fluffy like a cloud? Do you have a particular size in mind?
The pick of all synths I have used is the ApSc SynBad, I know there are a few fans of it here as well. It's incredibly soft, but the knot still keeps it's shape after being splayed and mashed around. It's a superb knot for face lathering, but a touch too soft for bowl lathering?
I've got a Boti N3C which I presume is a new generation like some others (maybe it's a bit like the Muhle?) and it's an excellent knot for building a lather with, but difficult to splay.
The run of the mill of the Tuxedo knot is a classic as well, I think of all the synths I have used I still think it's the best all rounder (face/bowl/sticks/creams, works perfectly every time). IMO, the Yaqi Sagrada is a bit of a classic and rightly so and it'd be my no.1 choice for starting with a synth. If you want something with more or less backbone you could take it from there. The Sagrada Tuxedo can be bought for about a tenner if you keep an eye out for their sales.