I've done the very thing you describe with my 3 gallon kegs in the past (assuming you are using the 12-16g cartridges mentioned in the post above). It doesn't hurt to purge the O2 at all, and it'll give you peace of mind that the keg is sealed.
What I did, gently rack to the keg, close the lid, and hit it with the CO2 with a 1-2 second blast with the relief valve closed.
Pull the relief valve to relieve all of the pressure.
Hit it again with CO2 (1-2 second blast).
At this point I'd decide if I had enough CO2 left in the cartridge to pull the relief valve and seal it up again. You'll have to judge yourself based on feel. If this is the first time doing this, you'll probably use more than one cartridge - consider it a learning experience.
I think if you stick with the 1-2 second blast, you won't trigger the PRV, I never have.
Assume the dispenser is a one time deal, it'll leak out over the course of weeks, so I would not leave it attached for that long. Use it to purge and then seal, then take it off.
Put it back on later (with a new cartridge) to dispense if you need to.
I would recommend one of these
if you're going to naturally carb in the keg. Let it get to about 30 psi at room temp (70 degrees) and it'll be good. If it climbs above, just relieve some of the pressure. You can use the same amount of priming sugar you use for bottling with this method, just make sure you bleed anything off over 30 psi. Let it stabilize at roughly 30 psi, and then chill it down. It'll take 2-3 weeks just like bottling. Keep in mind, if you do monitor the pressure with one of these, it'll be high at first from the initial seal, then it'll drop as the CO2 gets absorbed, then it'll climb again as the pressure builds.