I've built out many sets over the years. I do not do so any more except for a few very, very rare ones where just finding a single card is a challenge.
I'd say the best advice is to buy the condition you ultimately want to own when it comes to mainstream sets, but also do not overpay. Most sets are commodities, not rarities, so you can hold out for cards in the condition you prefer at a price you can handle, and that is how I like to do it. When I built my 1971 Topps set, for example, I tried the buy in bulk and upgrade route, but I found it disappointing and expensive to keep getting crappy Mays and Clemente cards that I had to then turn over. Eventually, I culled through thousands of cards at shows to build it. I say get it right the first time. Now, if you happen to find a set that looks perfect for you, by all means buy and upgrade. Happened to me once with a 1973 Topps hockey set. Usually, not.
The other advice I have is that if you are chasing after a truly rare set, don't be a condition snob, because it will not go well. You may as well not even start if your plan is to build an N300 Mayo set in near mint. I am perpetually trying to build a set of Adam Hats premiums and my cards range from nearly perfect to barely intact, but the cards are so rare that I am lucky to find one a year, so I take them as they are found.
Ditto set of 1904 boxing cabinets:
Not the greatest condition but that's how it goes.