I mostly build bikes up from the frame and fork. But I have a deep parts bin and rarely have to buy anything new; mostly it's a matter of mixing and matching to find compatible parts or fulfill some wacky vision I might have. I've often seen on this forum the advice to buy a complete bike and switch the parts over to your frame, which seems like a good idea if you don't have the parts in the first place. Otherwise, the info on checking for frame damage is important (beware the replacement fork!), and something that has an English-threaded bottom bracket and headset will likely give you the most amount of options. It's definitely lots of fun to build up a frame from scratch, essentially a blank canvas for you to express something or other.