Hi Ross,
I have ordered a set of two 510mm rails type HSR25 with four TA carriages. I haven't really settled on what geometry yet, but £90 for this set was too tempting - I can likely re-sell if they are not what I want.
I spent several hours pouring over the datasheets from various manufacturers and found THK particularly helpful - even though the TA carriages don't seem to be on their site (possibly discontinued or special order?) The most worrying thing I have seen so far is that linear guides seem to be extremely intollerant of mounting errors - less of a worry if only a single carriage is used.
I get the point about mounting the blocks and tracks upside down. Presently I am modeling various parts for my various options on Turbocad in the hope of settling on an optimum geometry soon.
Mike