I mean, it isn't impossible to find people willing to accept commissions for sprites and code, but the only time I've seen that successfully made was in the case of Emmanuelexe (
this post and
this post). No problem with asking how much people charge for their work in all the stuff you mention to have an idea of the potential budget needed for your project, but it's usually more effective to come and say "I have money" instead "How much do you guys charge?", at least when refering to passion projects from what I've seen.
I'm not a Doom artist, but if I were one, I would at least want to see some maps or concepts done in the project of whoever is contracting first. Or a proper guidelines/to-do list that gives off some sort of professionalism and trustworthiness. It could also help you to ask in other sites as well, so you don't feel stuck with people in just one forum.
In any case, I hope some of these advices be of help to you and good luck with your project!