I wouldn't recommend buying stock Fostex as your main model with the intention of modding later because you'll probably want something you can enjoy immediately. So if you decide to try Fostex, consider ZMF v1.
Besides that, it all really comes down to a matter of taste considering how generally good all those models are. If I were buying myself a closed model for mostly classical, I would buy SRH840 with the full intention of modding the headband. To some extent, that reflects my preference for piano and organ over orchestral as I think some of the others make orchestral music sound better.
Guess I'm going to have to flip a 4-sided coins...840/k550 or k545/HP100/ZMF