4. Should I tile the floor or leave the concrete exposed?
You can certainly leave the concrete floor as-is if it is in good condition. I, however, am a big fan of epoxy floor coatings for concrete. You can use a relatively expensive garage floor coating that seals perfectly and looks great or go with a cheaper one-part epoxy, or an even cheaper and easier non-epoxy floor/porch paint. Anything that seals bare concrete makes cleanup easier, and epoxy coatings are wonderfully smooth and durable if you are using a floor drain, not to mention quicker and easier to install than tile.
I love tile and it is still preferred wherever appearance and a lot of foot traffic are concerns, but for a grow room an epoxy-coated or even painted floor is a nice, functional touch. So this is really just about your preferences, budget, and desired effort level. If going with the bare concrete, I'd give it a good scrub down before beginning construction, for ease without concern about where water and detergent goes, then another good cleaning after painting and finishing the room, but it should be fine.
Would you guys recommend a steel door?
What are your alternatives, and why are you thinking steel? An insulated steel door provides good insulation and installs easily. A hollow-core wood door would be at the opposite end of the spectrum, with solid wood doors probably cost prohibitive. Pre-hung steel doors aren't too expensive come already sealed fairly well. In terms of security, if that's a concern, door choice isn't too important, since the lockset or frame will probably be the weak point if someone is determined to break your door down. Hopefully that scenario doesn't need to be on your radar!