Here is my tale of woe:
I live in a forty-something-year-old condominium in Southern California. The plumbing is the original copper (supply) and cast iron (DWV). My bathroom sink drain had an extra side vent for attaching a condensate outflow tube from my heat-pump air handler. About six years ago, that rusted out and I had a lot of water collecting under the sink. When I noticed the moldy smell and investigated, I discovered that the sink drain was rusted out as well and decided to change out the plumbing.

I succeeded in R/R-ing the faucet and removing the drain. However, when I attempted to install a new drain & pop-up assy, I discovered that the upper and lower basin flanges were so corroded that insufficient material remained to retain a new fixture. Pieces of the flanges were breaking off. So, unable to afford replacing the sink immediately, I just put a bowl in the sink and used it for a basin, dumping it into the toilet when it got full.

Last month, I went to IKEA and bought a new vanity and top, and a new faucet. I purposed to demo my bath vanity and install the new fixtures. I discussed the project with my friend, who is a retired master plumber. He said that there could be serious problems with a sink drain that has been disconnected from a fixture--and hence had no water running through it--for an extended period of time. As he is recovering from surgery and unable to come to my house or even speak at length about it, I didn't press him for details.

All I want to do is replace the angle stops for the supply and connect my new sink to the P-trap. What I'd like to know is, what sort of problems am I likely to encounter with that? I can anticipate the trapped-water plug in the trap having evaporated. However, as I have experienced no foul odor from escaping sewer gases, I'm not even sure that that has occurred. What else might I be looking at here? I should mention that this is a ground-floor unit with frame-on-slab construction.

there will be no problem with the hot and cold lines. if the drain (being cast iron) has some accumulation of hair and soap scum inside, it might have hardened slightly but should loosen when hot, hot water is run thru it.
if you are removing the trap, you might poke a 1/4 inch snake wire in there while it is open. otherwise, i would not worry.

Gas from sewage is the main problem. Rats or flies can come up.

Methane is not detectable. Just pour water in it or seran wrap and rubber band it to keep gas/flies out of the house.