Depends on what you want to do with your clone. Typically, if you want a shooter, get a new barrel, and if you want a museum quality clone with as many original parts as you can find, get a take-off barrel. If you find an original NOS barrel, then you get the best of both worlds, like the Mk14 Mod 0/1 barrels that sometimes pop up for sale.
Most of the original M40 barrels I have gauge pretty high, I think one over them was over 7 at the muzzle! You might get lucky and find a take-off barrel that isn't shot out, I have a few original M14 SSR barrels that gauge about 1.5 at the chamber and zero at the muzzle. Some take-off barrels are just pure unobtainium, like the M40A1, Mk13 Mod 3 and Mod 5 barrels, so the only route to go is using a newly produced barrel.
Matching paint always matters to clone collectors, all painted rifles are unique and the paint adds character. I sold over 100 original Mk13 Mod 0 stocks and the painted ones were the most desired by collectors, those kits sold out almost instantly. I still get people asking about painted Mod 0 kits even though those were gone years ago! There's a few M40A5 take-off parts kits floating around, the paint is what makes those kits interesting. All A5's pretty much look the same, green stock and black metal parts. The unique paint makes each one stand out and tells the story of that rifle.
You have to decide what you want out of your clone, how much you want to spend and how long you want to search for parts. Original, rare parts can get very expensive and some stuff can take years to track down. Unless you somehow have an original receiver, the build will always be a clone, no matter how many original parts you have on it. Additionally, your choice in a gunsmith is extremely important. There's no point in having rare, expensive original parts if they're sent to a gunsmith that doesn't have enough knowledge to build your rifle to original specs. Guys like Chris Higgins and Mark Williams have a ton of experience building various clones and they'll make your build perfect!
Just my 2 cents and what I've personally seen in the collector community, I hope some of this info helps you out