You should be able to migrate with no issues since you are keeping the same routed mode, just keep in mind that if you are using a single interface on ASA to create your four networks from that of the quad card of the PIX you will need to used 802.1q and create your vlans in the switch, other than that you should face no problems by buidling your configuration on the new ASA.
Personally I would go with your Plan1 design as it provides redundancy for your DZMvlans and outside connected hosts on your 3750G switches, you are also trunking etherchanneling between the 3750Gs correct?, I do have similar design using Active-Standby 515Es still and currently looking to migreate to ASA just as you are.. As for the IPS I am anble to comment because I have not yet used it but I would assume you are using IPS on both firewalls ? I would let someone comment on the IPS portion ..
AS for issues with protocols such as FTP, SMTP on 8.03 I am not aware since it is a brand new code, your best bet would be to check bug database on code 8.03 to see what bugs have been reported so that you are prepare, that is what I do when I have migrated to to version 7.0 on PIX, checking the bug database gives you beter perspective on opened bugs that one should be awaye prior to migration.
go to bug tool kit and check the current opened bugs on 80.3 if any.
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