definitely suspension first, IMO. its nice to be able to drive the car around and get a instant feel for the changes you've just made without having to wait a year or two while the car is de-motivated.
obviously the easiest swap would be the L28ET, plenty of people have done it so there is a vast amount of information available to you, dont forget that it will be easier on your wallet too, allowing you to enjoy the car sooner than later.
I would probably shorten the strut housings and install coilovers with your new adjustable struts (take the time to blast and coat the suspension parts while your at it, it wont take that much longer and will make the car more appealing visually and physically when it comes time to work on it. install some sway bars( dont forget to reinforce the front sway bar mounts. install an LSD with some kind of CV axle swap and then move onto a brake upgrade as the last phase of suspension work...
this is my opinion, take it how you will...
also, if your still undecided about the engine you want, buy a few raffle tickets for the pinks Zs engine...if you win youll have a multitude of less things to worry about.