Which version of VMware convertor you are going to use, and on which os the oracle is installed.
1) What would i need to concider when taking on this task (Best Practices)
refer the relese notes of the convertor for the OS compatibility
it is good to stop the DB instance because if the rate of change of DB is high then some corrouption may occur
if the OS if rhel 5.x or windows 2003, then you need to use the vmware covertor 5.1 so that you can align the partition, after the conversion it wont be a straight forward process and easy
2) How long would the P2V take per Gig of data
depending upon the size the network bandwidth the convertor machine speed etc... the storage performance etc.. lot of things there no single answer
3) Is there a way to minimize the down time
give more cpu and ram for the vmware converter machine.... and use a dedicated datastore in the destination and check the array performace.. that is do during the off peak hours.
4) Is there and alternative solution such as mirroring
it is easy and good to do a fresh installation...and restore the DB from a latest full backup... and then do a mirroring...by using oracle tools..