I don't think difference b/w mk2 and LCR is that big like it was b/w mk1 and mk2.
Also I don't recall if you need new crossover for beryllium twitter upgrade or not, I just recall that upgrade cost was too big (due to twitters price). So it's like if you are buying used 99.25 / 99.26 and able to score upgraded ones - you should get one, if not - just stick with ribbon twitters, they are pretty good.
99.25 and 99.26 were identical except cabinet form. As for 99.25 - all three (mk1, mk2, LCR) looked identical AFAIK.