It really totally depends on the employer.
I transitioned on the job and everyone was basically fine with it. They just took to calling me by my normal name, but doing only the paperwork in my birth name. If your employer knows you're trans (which is sort of inevitable if you haven't settled all the bureaucratic concerns), it doesn't mean he has to treat you any different. Just make it clear you don't want to be treated differently from any other male there if that seems to be happening.
I don't know if being trans would prevent you from getting hired; again, I think it would depend on the employer. You're probably pretty safe, though, if you go with a bigger corporation, as they usually have some kind of policy to avoid discrimination in general (though normally not specifically with regards to transsexuals, it still helps in the general spirit of things, I'd imagine). Besides, you're just an impersonal cog in their wheel, so they'll care less than a smaller business that you're trans.
All I can really say, whether you're transitioning on the job or just starting at a new one, is to act with confidence about everything. Just do things like you normally do them--use the men's restroom nonchalantly without asking permission for it, respond only to your real name, etc--and act confused if someone questions you about it, making it clear that this is normally how things are done and that it's weird to expect you to do things any other way. Usually that's not a problem, though, because I've noticed that people generally won't question you about stuff like that and you're free to do what you want so long as you don't ask for it or otherwise reveal any sort of uncertainty on your part.
Transgendered people aren't too commonplace and it's likely any employer you have doesn't know exactly how to deal with it all, so lead the way and tell them what to do, how they're supposed to treat you, etc. Since they likely have no other source of instruction on how to treat a transperson besides you, you'll become their only source and, in my experience, they'll just tend to go with whatever you say (if they're not ->-bleeped-<-s with pre-formed opinions about transsexuals, that is) since they have nothing else to go by.
So, yeah, in my experience, you never really have to fight anyone on anything if you just act like what you're doing is normal and don't seek approval for it (because seeking approval implies that you don't have a right to these things in the first place and casts doubt in their minds, puts the burden on them to decide what to do, when really it should be on you).
