If you're confident in peoples tolerance, you could simply tell them you're into girls and leave it at that. If you go into being transgender, people will probably question you more than they do now. Even tolerant, open-minded people don't really understand it.
I totally understand. Even though I know that where I live, homophobia is the exception, not the rule, I'm still hesitant to tell people I'm gay. The bad experiences just weigh so much more than the good ones.
I know telling people you're transgender will be even more nerve wracking, so I wish you the best with that.
"So yeah should I just keep going "nah nah not looking for a man" without explaining why I'm not looking for a man or just go out of my way and say the truth?"
I think the former's just easier for you. If you keep saying you're not looking for someone right now then yeah, people will continue to poke and prod you until you do - because to them, the whole "I'm not looking" phase is something that will come to pass.
But imagine the bullshit you'll have to deal with after your workplace colleagues finds out you're gay or bisexual. And that you're looking for a "pansexual girlfriend". And that you're FtM transgender. Giving them a serious answer will not sate their curiosity, it will only raise more questions. Not to mention the assumptions people will make about you because of your sexuality from that point on.