I use good quality camel hair brushes that I bought at an art supply store over 20 years ago... I expect they'll last as long as I will
For silk I will use the foam brushes for the first couple of coats until the weave begins to fill but I don't need them for silkspan.
One trick that will help is to run a little thin CA along the ferule prior to use- this will prevent most occurences of losing bristles with the cheapy brushes.
Although I've been guilty of it myself, I wouldn't recommend silkspan for a Seniorita. That nice stick built open structure would leave the silkpan very suceptable to punctures. Silk is a better (though more expensive) choice, but there are places that sell model grade silk at much better than hobby shop prices.