My builds generally revolve around what I think I can sell. I have been doing the flipping for profit for a little over a year and a half. I have learned a lot in that time (many thanks to the posters here!!). I can relatively easily look at a garage sale/thrift store bike figure out what it needs in new parts and what I think I can sell it for when finished. If it doesn't need that much or is worth it for parts I buy it.
The next part is my biggest problem. What to do with it? Most of the bikes I get I strip down to the frame and fork and clean/regrease headset/BB and then it just sits there. I find it hard to make up my mind with what I end up doing with a bike. Return it to stock? Single Speed? Fixed Gear? sell it as a Frame/Fork for easy profit? Or something else?
I will often change parts if the "original" part doesn't work well or is broken. I often change parts if they are higher end too. I have found many bikes are worth more in pieces than whole so I often remove nicer parts and sell those on CL or Ebay. I will then put on fully functioning but kind of basic parts to get the bike running again. I have done slight upgrades on a couple of occasions as well (aluminum rimmed wheels in place of Steel 5 speed to 6 speed freewheel, etc.).
These are all for flippers. For my own bikes I tend to take things differently. My bikes are setup to be usable by me. So many of my vintage bikes have Thomson seat posts because they are strong, look nice, and are long enough for me to ride. The other is I ride clipless Time ATAC MTB pedals on almost every bike. I know they aren't PC on a vintage bike, but I don't care. I generally don't care much about period correctness and setting everything up as it came. Mainly because I don't have to means to find and purchase the needed parts. My 73' Paramount is the one exception as it is almost completely stock and will stay that way because it is my one period correct bike. I purchased it and my 76 P14 from the original owner and the P13 was almost complete period correct except he switched out the tubular wheelset for clinchers. Both of my vintage track bikes are more along the lines as what works for me. One has a carbon fork that made me like to ride the bike again and a Paul/Velocity Aerohead wheelset. The P14 has Phil Wood/Mavic MA3 wheelset. I couldn't bring myself to put a set of Deep Vs on the Paramount so I seeked out a nice set of track wheels with older box section rims. My other geared bike is a mid-80's Eddy Merckx Corsa with slightly later Shimano 7 speed brifter group. I bought it basically that way off of Ebay, and I think I might eventually make it a full Dura Ace group. I bought it because I wanted an older lugged steel road bike with newer STI. It isn't a full 9/10 speed Dura Ace or Record group, but for the price it has filled that need at least temporarily.
The one bike I want to add to my collection is a touring bike. I would really like to find a Schwinn Paramount P15 to add to my Paramount collection. I would probably keep that close to original and PC as possible. Even if I did I would consider a newer touring bike for longer rides. I know it isn't C&V, but I would buy one of the complete Surly Long Haul Truckers if I had the money in a heartbeat. I would upgrade a few things on it like adding a Brooks saddle of course. A Rivendell would be nice, but the LHT complete is the price of many of there frames/forks.