I built up most of my bikes from parts.
But, I am retired, have the spare time available for searching out the parts I want for the best price.
And I worked in a bike shop years ago, not afraid of getting into a project like that. I own most of the tools that I need. The only tools I lack are the ones to press a headset onto a frame or fork, I have paid bike shops to do that. I do not have a truing stand, but am competent at truing up a wheel in a frame using the brake blocks.
There is a lot to building up a bike. Example, just the wheels, you have the hubs, spokes (of correct length), nipples, rim tape, lots of things to buy. And then you need to know how to build the wheels. You can buy prebuilt wheels, that saves a lot of time.
It often is not a big money saver, but if you have specific ideas on what components you want to have, building it yourself is the best way to accomplish that. Or, hire someone to build it up from parts, but that gets expensive with the labor costs.
If you buy a used frame, get the frame in hand with a close inspection for making sure it is not bent or otherwise unusable before you spend money on anything else.
I would be surprised if it is 27 inch for tire size, canti brake posts became much more common after the switch to 700c. That said, if it was 27 inch, I would not buy it, tires are too rare.
If you want to tour on it and want a front rack, there are options for that but they can be expensive. Or, you might get lucky and find a used fork that has the right axle to fork crown length and right fork rake with enough steerer tube to make it work.
One of my bikes has 130mm dropout spacing in back, I am running a 135mm hub in that frame. I talked to the manufacturer and they said that it would be no problem, other than dropping the wheel into the frame takes a bit more work to open up the frame a bit so that it fits together.
If the downtube is the vintage diameter where the downtube is the same size as teh top tube and seat tube, that frame might be a bit more flexible than you would want for touring. Newer touring bikes usually have bigger downtubes for a stiffer frame.