As with evaluating all product functionality we recommend that you first try the search/index on a small subset of data.
The "Invalid Index (0x8004160a)" is normally concerned with the index not yet being ready to search, is it possible that the index hadn't yet been created? If you'd like more information please click Help->Generate Error Report.
The key to speeding up indexing is both the speed of access to the source data and also the speed of access to the index database location. If both are located on network drives then the index process will be much slower than when they are located on local drives, especially if those local drives are SSD based (a recent benchmark test showed 200,000 emails without attachments being indexed in just over 150 seconds).
Obviously indexing complex formats like PDF will also take much longer than normal text files.
Finally, the amount of RAM on the indexing computer also helps as it reduces the number of 'flushing' that needs to be done.