Introducing a puppy may bring problems to your current dog because a pup has got way higher energy levels than an older dog and the pup will want to play a lot which may cause stress for the older dog. What age pup are you getting? Is your dog particularly territorial and is he socialised properly? These are things to consider when thinking of getting another dog. If you do go ahead and purchase a puppy, it would be wise to get a female pup, because a male is likely to try to dominate the older dog when he reaches puberty and being younger and stonger is likely to succeed which is unfair on your dog.
It would be wise to introduce the dogs on neutral ground, away from your place of residence and if possible, off the lead. Having the dogs meet at home where your dog has long-ago established his territory may cause an aggressive reaction from your dog which again, is unfair on your dog and the pup.
Apartment living creates less space which is also stressful on your dog. Getting a gate to separate the dogs while you are away is not fair on the pup. Puppies love and need to play and if it doesn't have the opportunity to do so, is likely to focus his attention on other things and could become destructive and/or howl and bark out of frustration of not having access to your dog. Having toys may keep it entertained for a while but another dog in the apartment is far more appealing than a few toys.
Having said all that, my advise is to avoid the situation altogether and not purchase a puppy. Is is likely your dog would be far less happier having a playful, energetic pup around at his age than the status quo.