jpc,
not sure what you mean by "ordering products"............
I am only aware of Weaver offering factory weathered O gauge rolling stock.
I have some of their factory weathered tank cars, which are very nice.
"my concern is matching or blending products from different suppliers".......
If you have some items weathered and some not, well, yes, the differance will stand out.
Most 3 railers do not weather any trains. Some do , but most do not. If your worried about one weathered item next to another weathered item, nothing to worry about there. Just look at the real trains. You'll see anything and everything next to each other.
I do not have the skills at this point to do it myself....................
Take it from me, it's not that hard. I got myself a book or two from my local Hobby shop that specializes in HO, watched a few videos and I gave it a shot. Start simple, perhaps a simple chalk set and some brushes. that'll get your interest peaked. Ask your local HO hobby shop or visit a local NMRA meeting, they are always willing to teach.
is best to give up the authenticity and run it "out of the box new"...........
That's something only you can answer. Doesn't hurt to weather a few inexpensive cars (or purchase them) and see what you think. To me, my collecting days are over, I'm all into not building a "layout" but building a "railroad". Meaning that the railroad will have meaning in the design, serving a purpose as a real railroad would, so, shiny new out of the box stuff doesn't look good on my railroad. Everything gets weathered. Most things very light.
Sometimes even a simple spray of Testors Dullcote on a shiny new car can tone it down to an acceptable level.