I think if you decide to go down the weight weenie path, then you must be prepared to do all the building/maintenance yourself. It is actually not difficult. I also don't really trust the bike shops to correctly tension items. Whenever you have been at LBS near the workshop how often have you seen a torque wrench in someones hands? Never?
Yea, I was really surprise that anyone who is interested enough in this hobby would be using a bike shop for installation of parts, especially such dead simple stuff as stems, seatposts, etc. "Just Riders" type of folks, yea, I understand that. All they want to do is ride the damn thing and could care less about how it works. Truly nothing wrong with that.
But they aren't interested in the arcane details of component weights. People who are have no excuse for using a shop for their work.
Folks, there is nothing, absolutely nothing, about assembling a bike that a person with no mechanical background cannot do with a simple "how to" book and/or a decent competence at finding online resources.
The tools are virtually trivial, most are just standard allen keys - although all the standard and basic specialty tools could cost a couple hundred US dollars all in all, or about .75 as many Euros.
Aside from facing and chasing, pressing some bottom bracket bearings, and aligning a RD hanger (I do get those things done at a shop, but that isn't part installation) the tools are simple hand tools, mostly standard tools. The few specialty tools you need can be found in non-commercial quality from places like Pedros, Park or Performance for very little money.
I have almost everything I need to completely assemble a modern or retro bike (don't have bearing presses, BB facer or hanger aligner) and I probably have $300 in tools, and $150 of that is the repair stand alone. I even have a couple of beam torque wrenches, a variety of BB tools and way more than necessary redundantcy in my hex/allen keys (L shaped, T-handle w/ ball end, replaceable bit screwdriver handle, etc.).
I can do almost every installation and adjustment on the road with my mini-tool.
I'm no mechanical genius but learned this stuff all on my own, in the old days using "Glenn's" comprehensive bicycle repair manual, and currently just looking for specific things online.