I've only ever went thorough the bricklink process once about 5 years ago to recreate a 391 Renault and 395 Rolls Royce which were rather simple projects compared to the task I've now set myself. Having resigned to never being able to get my hands on a Taj Mahal (I simply just don't think that the price of even a used set is now worth it), I decided to start bricklinking one at the weekend.
I'm still at the stage of finalising my "to buy" inventory. I've found the process of going through which existing pieces I already have to be very time consuming, as I'm studying the manuals in close detail to work out which pieces can be substituted. While I verge on OCD on some things, I'm not being a purist on this one and have decided to substitute non-visible elements (usually colour, but sometimes the element itself) where I can do so using my existing pieces or the cost of buying them will be reduced. I don't feel so bad about doing this as Lego routinely has non-visible pieces in other colours in many sets, including the Taj Mahal itself.
What are people's views on freely substituting non-visible pieces?
It looks like I already have about once third of the required elements so now the arduous task of obtaining the rest follows. I'll be interested to know how the remaining 2/3 will cost me! I'm looking forward to progressing this project.