If it just one or two parts prob worth getting the full bike.
BUT depend on how custom you want to go when building a custom bike, you can choose a colour scheme (BB, Hubs, headset spacers, seat clamp, cables etc) that does not cost any extra than a non colour part, and it make the bike look loads better IMO.
Also, As above look at cost of a frame, and cost of bottom level full bike with basic kit, strip the bike sell parts, or even put it on a old frame/bike to sell or use as a winter bike.
On the lack of maintenance front, look into doing a bike maintenance course, I do some set courses and also can custom them a bit to want the customer wants, and I have done a few "bike build" courses, where we used the customer bike/parts etc, it does cost a bit more than just me charging to just building up the bike, but then the customer is also learning how to do it and how to maintain the bike in future. I sure a good LBS would help with this. (might have to pre book in advance)
Also if you chat/talk to the LBS and get ALL parts through them, I am sure the "build" cost would be less than
"here's all the parts I got cheap online, I hope they all fit and the right standards to fit the bike, please build"
Plus they have to make sure the parts are the right standard and maybe help with choice of kit to suit you and your type of riding.
Also by getting the parts from a good LBS it will save you a lot of time and effort if something goes wrong with any parts.