I beleive you'll be restricted in operating altitude even with Mode S unless you also have Enhanced Surveillance (EHS). EASA's just written an open letter on Mode S that you can read here:
http://www.easa.eu.int/doc/Certifica...let_to_ind.pdf
I would expect that dual transponders are also required - even though only one is transmitting at any time you need the other as back-up in case of failure (this is more likely as EHS requires quite a lot of different parameters to be downlinked to ATC over the Mode S some of which come from your FMS). You'll probably also need to be RVSM-capable to avoid the restrictions as well.