cross platform migration

I have a project that is going to get started soon and i wanted to get some Pointers with regards to it. Please excuse my knowledge, as i am from SQL Server background and a seasonal oracle DBA. Project is to move our DB(multiple DB size from 1TB - 30TB) from hp-ux pa risc to RHEL. We have quite a few restriction in options as our app is SAP :-( . Couple of SAP notes i read suggested that we can use use cross platform transportable tablespace, which is what i am planning to do as well. I wanted to find out couple of things from the list

1. First of all if anyone has done this(on a SAP system), if so any gotcha
2. To reduce the down time i was planing to do a restore ahead of the cut over(lets say 3 days in advance) and then keep applying archive log until the day of cut over. Is that even possible for this situation (as i have to do RMAN Convert of the datafiles and then keep applying logs)
3. i cannot do transportable DATABASE, as i am going from big endianess to little( i believe #2 is possible here, as i read this doc<http://www.pythian.com/blog/howto-oracle-cross-platform-migration-with-minimal-downtime/> )
4. I read an option some place that mentioned i could have heterogeneous data guard setup for this migration, but when i read MOS doc ID 413484.1, i do not think hp ux to RHEL Data guard is supported or have i gotten that wrong.
5. any other recommendation in general due to the size of our DB. The one i am worried about is our 30TB DB which takes about 18 hours to do weekly Level 0 backup and customer wants to do the migration in less than 10 hours
6. is cross platform transportable tablespace a bad idea as SAP creates thousands and thousands of objects in the database and the metadata would be too much to export/import

I would really appreciate some pointers.

Thanks,
Max

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