I had a slight advantage in the fact I met with a couple of members from here and had a go on their bocks, a set of m90's and a set of m100's too, I found that although the springs were obviously tougher on the 100's I wouldn't say you would find them rock solid.
If you are already fairly fit and healthy then you should be able to practice longer and get the techniques down quicker, plus you said your more likely to gain weight and move into the next bracket.
I'm a complete noob too but it sounds to me like you may as well go for M100's because if over time you do gain weight or technique, then you will need them.
I've just ordered mine (M90's) because the last time I did some serious sport was a few years ago, unless you count motorcycle racing.
So I'll be regaining lost fitness at the same time, although I fully expect to have to invest in some M100's in a year, maybe less.
My opinion, not to be taken as direction.
