^ Agree 100% on starting with something you love (and really, adding to that more things you love). Everyone says the things you love work together, and I've found that to be true.
So far I've done everything myself, KOV ... and for my bedroom, I kind of had a plan. I have a mix of new pine/maple furniture I've bought (mostly when I was younger--now I buy new only upholstery, beds, recliners, office furniture), vintage Art Moderne birdseye maple, a vintage painted + oak piece, and a couple other painted things. What I thought was a little risky is I wanted to use some red in my color scheme and almost no one does that in bedrooms. I did it & it worked perfectly, I love it. So you're right, when you're not a professional, it's time consuming and a little scary and slow, but so rewarding.
Btw, I have bought a number of items from 1stDibs (maybe a dozen or so?), I just do one or two at a time. They do have some relatively affordable things. I like Quatrine for upholstery, Thos Moser for new wood pieces (I've ordered a standing desk that's going to be finished this summer, that's not a vintage item!), I've bought from eBay before (including tables, the birdseye maple set, light fixtures), 1stDibs for statement vintage/antique pieces, and I've also gotten a number of affordable pieces from local antique malls--one dealer in particular who had booths at different malls I bought several pieces from.
Empty space is beautiful ... I think I still have less furniture in my living room than the previous owners had, and I think it's full.
I think I'm about to use a professional designer for the first time, for my bathroom. I have water damage and it's going to have to be gutted, so we can't futz around, and everything really needs to work. I'll know for sure tomorrow whether this problem originated with hail damaging the ridge of my roof, but of course the whole room needs to be designed to properly withstand normal levels of moisture. I'm excited about it because there are elements of this room I really dislike and now I'm being forcibly handed an opportunity to change them.