Are you planning on broadcast spraying or spot treating stumps individually? Fall is a great time to spot treat, as stated above the trees are putting energy into the roots so they really absorb the herbicide. Triclopyr and Tordon work great (I really like Triclopyr with diesel for basal bark treating) but I generally just use Glyphosate on cut stumps because it works and it's cheap.
A paint brush works fine but I highly recommend the Buckthorn Blaster (I got mine from
here). I have 4 of them and one always stays in the saw box and another on the UTV since Buckthorn, Siberian elm, White Mulberry and other invasive are everywhere. Normally I just mix glyphosate and water but when it gets cold I like to mix glyphosate and diesel, they don't mix well (water and oil) so you have to shake it once in awhile. Make sure you get some Mark-it-Blue too, makes it easy to see your coverage and see which stumps you've treated even a year later. Also, make sure you apply to every stump, especially on Buckthorn, if a stump has the tiniest side shoot it will continue to grow if untreated.