Mmm, just to chime in as well with my own experience.
Like p4 said, pants are a great foundation to start with. Some nice, wool gab tapered pants in your appropriate size will go with almost anything else you pick up from Yohji and should still be possible to wear with your existing shirts at least even if not jackets.
Personally, I went pants -> shirts -> blazers and then started accumulating other pieces at random and more unique pieces. If at all possible, I would probably advise that you pick up pieces in wool gab (fabric code 100) as much as you can. This is the classic Yohji fabric and can work for multiple seasons (unless you live somewhere really cold) with extra layers as necessary.
Funny enough, I also wear a lot of my first Yohji pieces a lot and they are probably all among my favourite pieces of clothing. I wonder why that is...
Anyway, if you can find somewhere to try on the clothing then that is definitely a plus, but I wouldn't be too wary of picking up a pair of pants online in an appropriate size. The thing about Yohji is to not care too much about fit etc. as has been said loads of times on here. You can see that dantebykiko for instance buys clothing in all sorts of sizes and it still looks great on him. Feel free to buy a really long belt, suspenders, roll up the sleeves of jackets, roll up the hems of pants, leave stuff dragging on the ground, use safety pins to close or cinch stuff etc. it is all part of it
Having typed all that, you should definitely take into account your own body type (height mainly, but build does come into this) and most importantly what makes you comfortable.