M & S make some amazing suits for good prices, you just have to know what to look for, no way burtons or next are anywhere near as good.
As the owner of Burtons/Top Man said a few years back, "the only people who buy our suits are those who are due in court the next day".
And as for M&S turning you into an old man, its a suit...hardly everyday clothes, you could buy a suit from M&S or Hugo Boss and it would make no difference, the principle is all the same.
Bought plenty of suits from M&S and always had great service as long as you go into one of the bigger stores with dedicated suit people (M&S Marble Arch is the best). End of the day you get what you pay for, one of their £100 suits will be pretty basic, go up in the range and you'll find some nice outfits.